Compressible Objects Having A Predetermined Internal Pressure Combined With A Drilling Fluid To Form A Variable Density Drilling Mud

ABSTRACT

A compressible object is described that may be utilized in drilling mud and with a drilling system to manage the density of the drilling mud. The compressible object includes a shell that encloses an interior region. Also, the compressible object has an internal pressure (i) greater than about 200 pounds per square inch at atmospheric pressure and (ii) selected for a predetermined external pressure, wherein external pressures that exceed the internal pressure reduce the volume of the compressible object and wherein the shell being designed to reduce localized strains of the compressible object during expansion and compression of the compressible object.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application under 35 U.S.C. § 120 ofInternational Application Number PCT/US07/10905, entitled “COMPRESSIBLEOBJECTS HAVING A PREDETERMINED INTERNAL PRESSURE COMBINED WITH ADRILLING FLUID TO FORM A VARIABLE DENSITY DRILLING MUD,” filed on 4 May2007, which claims the benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application 60/811,620, entitled “COMPRESSIBLE OBJECTSHAVING A PREDETERMINED INTERNAL PRESSURE COMBINED WITH A DRILLING FLUIDTO FORM A VARIABLE DENSITY DRILLING MUD,” filed on 7 Jun. 2006.Additionally, this application is a continuation-in-part application ofpending U.S. patent application Ser. No. 11/441,698, entitled “VARIABLEDENSITY DRILLING MUD,” filed on 25 May 2006, which is a continuationapplication under 35 U.S.C. § 120 of International Application NumberPCT/US/05/20320, entitled “VARIABLE DENSITY DRILLING MUD,” filed on 9Jun. 2005, which claims the benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/580,523, entitled “VARIABLE DENSITYDRILLING MUD,” filed on 17 Jun. 2004. Each of these applications ishereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

This invention relates generally to a method to enhance drilling andproduction operations from subsurface formations. More particularly,this invention relates to a method for selecting, fabricating and usingcompressible objects with a drilling fluid to form a variable densitydrilling mud that minimizes or eliminates the number of different sizedcasing strings utilized within a wellbore.

BACKGROUND

This section is intended to introduce the reader to various aspects ofart, which may be associated with exemplary embodiments of the presentinvention, which are described and/or claimed below. This discussion isbelieved to be helpful in providing the reader with information tofacilitate a better understanding of particular techniques of thepresent invention. Accordingly, it should be understood that thesestatements are to be read in this light, and not necessarily asadmissions of prior art.

The production of hydrocarbons, such as oil and gas, has been performedfor numerous years. To produce these hydrocarbons, a wellbore istypically drilled in intervals with different casing strings installedto reach a subsurface formation. The casing strings are installed in thewellbore to prevent collapse of the wellbore walls, to prevent undesiredoutflow of drilling fluid into the formation, and/or to prevent theinflow of fluid from the formation into the wellbore. Typically, theprocess of installing casing strings involves tripping, running casing,and cementing the casing strings. Because the casing strings in thedifferent intervals pass through already installed casing strings, thelower intervals of the casing strings typically have smaller diameters.In this manner, the casing strings are formed in a nested configurationthat continue to decrease in diameter in each of the subsequentintervals.

In addition to the casing strings, a drilling mud is circulated withinthe wellbore to remove cuttings from the well. The weight or density ofthe drilling mud is typically maintained between the pore pressuregradient (PPG) and the fracture pressure gradient (FG) for drillingoperations. However, the PPG and FG increase along with the truevertical depth (TVD) of the well, which present problems for maintainingthe drilling mud weight. If the weight of the drilling mud is below thePPG, the well may take a kick. A kick is an influx of formation fluidinto the wellbore, which has to be controlled for drilling operations toresume. Also, if the weight of the drilling mud is above the FG, thedrilling mud may leak off into the formation. These lost returns resultin large volumes of drilling mud loss, which has to be replaced for thedrilling operations to resume. Accordingly, the casing strings areutilized to assist in maintaining the weight of the drilling mud withinthe PPG and FG to continue drilling operations to greater depths.

With subsurface formations being located at greater depths, the cost andtime associated with the forming the wellbore increases. For instance,with the nested configuration, the initial casing strings have to besufficiently large to provide a wellbore diameter of a specific size forthe tools and other devices near the subsurface formations. As a resultthe diameter of the initial casing strings is relatively large toprovide a final useable wellbore diameter. The large diameter increasesthe costs of the drilling operations because of the cost associated withthe increased size of the casing string, increased volume of cuttingsthat have to be managed, and increased volume of cement and drilling mudutilized to form the wellbore. As such, the cost of typically drillingoperations results in some subsurface formations being economicallyunfeasible.

To reduce the diameter of casing strings, various processes areutilized. For example, drilling operations may utilize variable densitydrilling mud to maintain the drilling mud within the PPG and FG. Asnoted in Intl. Patent Application Publication No. WO 2006/007347 toPolizzotti et al., compressible objects may include compressible orcollapsible hollow objects of various shapes or structures. Thesecompressible objects, which are selected to achieve a favorablecompression in response to pressure and/or temperature changes. Thesecompressible objects may be recirculated as part of the variable densitydrilling mud to provide volume changes that reduce the number ofintermediate casing string intervals in the wellbore.

However, the use of compressible objects in the variable densitydrilling mud can be challenging. For instance, the compressible objectshave to be fabricated to provide a certain amount of compression and tobe resilient. Further, the compressible objects have to be designed tocompress at certain pressures to provide the volume changes in specificintervals within the wellbore. In addition, the drilling fluid, which iscombined with the compressible objects, may be selected and includecertain additives to interact with the compressible objects to enhancethe variable density drilling mud. As such, there is a need for a methodfor selecting and fabricating compressible objects for use with drillingfluids to form the variable density drilling mud.

Other related material may be found in at least U.S. Pat. No. 3,174,561;U.S. Pat. No. 3,231,030; U.S. Pat. No. 4,099,583; U.S. Pat. No.5,881,826; U.S. Pat. No. 5,910,467; U.S. Pat. No. 6,156,708; U.S. Pat.No. 6,422,326; U.S. Pat. No. 6,497,289; U.S. Pat. No. 6,530,437; U.S.Pat. No. 6,588,501; U.S. Pat. No. 7,108,066; U.S. Patent ApplicationPublication No. 2005/0113262; U.S. Patent Application Publication No.2005/0284661; and Intl. Patent Application Publication No. WO2006/007347.

SUMMARY

In one embodiment, a compressible object is described. The compressibleobject including a shell that encloses an interior region, wherein thecompressible object has an internal pressure (i) greater than 200 poundsper square inch (psi) at atmospheric pressure and (ii) selected for apredetermined external pressure, wherein external pressures that exceedthe internal pressure reduce the volume of the compressible object andwherein the shell is designed to compensate for localized strains of thecompressible object during expansion and compression of the compressibleobject. The internal pressure may also be greater than 500 pounds persquare inch at atmospheric pressure, greater than 1500 pounds per squareinch at atmospheric pressure, or greater than about 2000 pounds persquare inch at atmospheric pressure. Further, the internal pressure maybe in a range from 200 psi up to the tensile strength of the shellmaterial at atmospheric pressure, in a range from 2000 psi to thetensile strength of the shell material at atmospheric pressure, and/orin a range from 1500 psi to 3500 psi at atmospheric pressure.

In a first alternative embodiment, a drilling mud is described. Thedrilling mud including compressible objects, wherein each of at least aportion of the compressible objects has an internal pressure (i) greaterthan 200 pounds per square inch at atmospheric pressure and (ii)selected for a predetermined pressure, wherein external pressures thatexceed the internal pressure reduce the volume of the compressibleobject wherein the shell is designed to compensate for localized strainsof the compressible object during expansion and compression of thecompressible object. Further, the drilling mud includes a drillingfluid, wherein the density of the drilling mud changes due to the volumechange of the compressible objects in response to pressure changes asthe drilling fluid and compressible objects circulate toward the surfaceof a wellbore.

In a second alternative embodiment, a method associated with drilling awell is described. The method includes selecting compressible objects,wherein each of at least a portion of the compressible objects has aninternal pressure (i) greater than 200 pounds per square inch atatmospheric pressure and (ii) selected for a predetermined externalpressure, wherein external pressures that exceed the internal pressurereduce the volume of the compressible object; selecting a drillingfluid; introducing the compressible objects to the drilling fluid toform a variable density drilling mud, wherein the variable densitydrilling mud provides a density between a pore pressure gradient and afracture pressure gradient for at least one interval of a well as thevariable density drilling mud circulates toward the surface of the well;and drilling a wellbore with the variable density drilling mud at thelocation of the well. Further, once the wellbore is formed, hydrocarbonsmay be produced from the wellbore.

In a third alternative embodiment, a method for forming a variabledensity drilling mud is described. The method includes selectingcompressible objects, wherein each of at least a portion of thecompressible objects has an internal pressure (i) greater than 200pounds per square inch at atmospheric pressure and (ii) selected for apredetermined well pressure, wherein external pressures that exceed theinternal pressure reduce the volume of the compressible object;selecting a drilling fluid to be combined with the compressible objects;blending the compressible objects with the drilling fluid to form avariable density drilling mud, wherein the variable density drilling mudmaintains a density between a pore pressure gradient and a fracturepressure gradient for at least one interval of a well as the variabledensity drilling mud circulates toward the surface of a well.

In a fourth alternative embodiment, a system associated with drilling awellbore is described. The system includes a wellbore; a variabledensity drilling mud disposed in the wellbore, wherein the variabledensity drilling mud has compressible objects and a drilling fluid,wherein each of at least a portion of the compressible objects has aninternal pressure (i) greater than 200 pounds per square inch atatmospheric pressure and (ii) selected for a predetermined wellpressure, wherein external pressures that exceed the internal pressurereduce the volume of the compressible object. The system furtherincluding a drilling string disposed within the wellbore and a bottomhole assembly coupled to the drilling string and disposed within thewellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the present technique may becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is an illustration of an exemplary drilling system in accordancewith certain aspects of the present techniques;

FIGS. 2A-2D are an exemplary chart and embodiments of a compressibleobject in accordance with aspects of the present techniques;

FIGS. 3A-3C are exemplary embodiments of a compressible object indifferent states in accordance with aspects of the present techniques;

FIG. 4 is an exemplary chart of different shaped compressible objects inaccordance with aspects of the present techniques;

FIG. 5 is an exemplary flow chart of the selection and use of a variabledensity drilling mud for the drilling system of FIG. 1 in accordancewith certain aspects of the present techniques;

FIG. 6 is an exemplary flow chart of the selection and fabrication ofcompressible objects for the flow chart in FIG. 5 in accordance withcertain aspects of the present techniques;

FIG. 7 is an exemplary chart relating to the shape of compressibleobjects in accordance with certain aspects of the present techniques;

FIGS. 8A-8B are exemplary embodiments of fabrication processes utilizedin the flow chart of FIG. 6 in accordance with certain aspects of thepresent techniques;

FIG. 9 is an exemplary flow chart for a fabrication process utilized inthe flow chart of FIG. 6 with compressible objects having a foamtemplate in accordance with certain aspects of the present techniques;

FIG. 10 are exemplary embodiments of compressible objects fabricatedfrom the flow chart in FIG. 9 in accordance with certain aspects of thepresent techniques;

FIGS. 11A-11B are exemplary embodiments of fabrication processesutilized in the flow chart of FIG. 6 in accordance with certain aspectsof the present techniques;

FIGS. 12A-12C are embodiments of a compressible object having a flangein accordance with aspects of the present techniques; and

FIG. 13 is an exemplary chart relating to the addition of a flange tothe compressible object in accordance with certain aspects of thepresent techniques.

DETAILED DESCRIPTION

In the following detailed description and example, the invention will bedescribed in connection with its preferred embodiments. However, to theextent that the following description is specific to a particularembodiment or a particular use of the invention, this is intended to beillustrative only. Accordingly, the invention is not limited to thespecific embodiments described below, but rather, the invention includesall alternatives, modifications, and equivalents falling within the truescope of the appended claims.

The present technique is directed to a method, composition and systemfor selecting, fabricating, and utilizing compressible objects in avariable density drilling mud. In particular, the compressible objectsmay be utilized with a drilling fluid to form the variable densitydrilling mud for drilling operations in a well. The compressible objectsand the drilling fluid are selected to maintain the drilling mud weightbetween the pore pressure gradient (PPG) and the fracture pressuregradient (FG) within a wellbore. Specifically, under the presenttechniques, the compressible objects have an internal pressure greaterthan about 200 pounds per square inch at atmospheric pressure, greaterthan about 500 pounds per square inch at atmospheric pressure, or morepreferably greater than about 1500 pounds per square inch at atmosphericpressure. The compressible objects may include compressible orcollapsible hollow objects of various shapes, such as spheres, cubes,pyramids, oblate or prolate spheroids, cylinders, pillows and/or othershapes or structures, which are selected to achieve a favorablecompression in response to pressure and/or temperature changes. Also, asdiscussed below, the compressible objects may include polymers, polymercomposites, metals, metal alloys, and/or polymer or polymer compositelaminates with metals or metal alloys, which are fabricated in a varietyof methods. Accordingly, various methods and systems are described toselect and fabricate the compressible objects. Further, it should benoted that the following methods and procedures are not limited todrilling operations, but may also be utilized in completion operations,or any operations benefiting from variable density fluids.

Turning now to the drawings, and referring initially to FIG. 1, anexemplary drilling system 100 in accordance with certain aspects of thepresent techniques is illustrated. In the exemplary drilling system 100,a drilling rig 102 is utilized to drill a well 104. The well 104 maypenetrate the surface 106 of the Earth to reach the subsurface formation108. As may be appreciated, the subsurface formation 108 may includevarious layers of rock that may or may not include hydrocarbons, such asoil and gas, and may be referred to as zones or intervals. As such, thewell 104 may provide fluid flow paths between the subsurface formation108 and production facilities (not shown) located at the surface 106.The production facilities may process the hydrocarbons and transport thehydrocarbons to consumers. However, it should be noted that the drillingsystem 100 is illustrated for exemplary purposes and the presenttechniques may be useful in circulating fluids in a wellbore for anypurpose, such as performing drilling operations or producing fluids froma subsurface location.

To access the subsurface formation 108, the drilling rig 102 may includedrilling components, such as a bottom hole assembly (BHA) 110, drillingstrings 112, casing strings 114 and 115, drilling fluid processing unit116 for processing the variable density drilling mud 118 and othersystems to manage wellbore drilling and production operations. Each ofthese drilling components is utilized to form the wellbore of the well104. The BHA 110 may include a drill bit and be used to excavateformation, cement or other materials from the wellbore. The casingstrings 114 and 115 may provide support and stability for the access tothe subsurface formation 108, which may include a surface casing string115 and an intermediate or production casing string 114. The productioncasing string 114 may extend down to a depth near or through thesubsurface formation 108. The drilling fluid processing unit 116 mayinclude equipment that may be utilized to manage the variable densitydrilling fluid. For example, the drilling fluid processing unit 116 mayinclude shakers, separators, hydrocyclones and other suitable devices(e.g., as described in International Patent Application No.PCT/US2007/003691, filed 13 Feb. 2007.

During drilling operations, the use of a variable density drilling mud118 as a drilling mud allows the operator to drill deeper below thesurface 106, maintain sufficient hydrostatic pressure, prevent an influxof formation fluid (gas or liquid), and remain below an FG that thesubsurface formation 108 can support. As noted in Patent ApplicationPublication No. WO 2006/007347 to Polizzotti et al., which isincorporated by reference, compressible objects may preferably have acompression ratio that is tailored to create a mud weight that liesbetween the pore pressure gradient (PPG) and the fracture gradient (FG)over the depth interval specific to the drilling application. That is,the compressible objects should have substantially recoverable loadbearing walls and low permeability for the gas within the compressibleobjects. Substantially recoverable is defined to mean that theaccumulation of plastic strain in the shell wall as a consequence ofrepeated cycling of the compressible objects between the surface and thebottom of the wellbore does not cause substantial failure of the loadbearing wall or significant loss of the internal gas pressure duringrepeated cycles (i.e. two or more cycles) as the well is drilled to thetarget depth. Also, low permeability is defined to mean that theinternal pressure of the compressible objects, while in use, remainswithin acceptable limits for a predetermined time period required todrill the wellbore to the target depth.

While adding compressible objects to drilling mud to control the densityof the drilling mud based on depth has been described in PatentApplication Publication No. WO 2006/007347 to Polizzotti et al., thedesign of compressible objects and selection of a drilling fluid toprovide this functionality is difficult. In particular, the repeatedcompression cycles typically experienced by a recirculating variabledensity drilling mud within the constraints imposed by the mechanicalproperties of existing materials may be a limitation for thecompressible objects. As such, the process of fabricating thecompressible objects may have to include various factors that influencethe durability and performance of the compressible objects, as discussedfurther below.

To begin, it should be noted that large compression ratios are requiredto achieve the desired change in the drilling fluid density with depthwithin the limits set by the maximum volume fraction of the compressibleobjects allowed by the effect of the compressible objects on the fluidrheology, as described in Patent Application No. WO 2006/007347.Accordingly, the compressible objects should have certain propertiesconfigured to provide large compression ratios and to begin compressionwithin certain pressure ranges or levels. The compression ratio of ahollow object, which is one embodiment of the compressible objects, maybe limited by the ratio of the initial uncompressed volume (i.e.uncompressed or expanded state) divided by the volume occupied by thematerial comprising the shell wall plus the volume of the compressed gasinside the shell for the delta pressure ΔP of the wellbore interval ofinterest. Large compression ratios are provided by the wall of thecompressible objects being thin and flexible. Accordingly, thecompressible objects may preferably be designed such that thecompression and re-expansion of the compressible objects may beaccomplished without significant permanent deformation of the walls(i.e., permanent deformation leading to early fatigue failure of thewalls of the compressible object).

In addition, the predetermined external pressure or depth of compressionand the predetermined compression interval of the compressible objectsmay be tailored to provide a change in the density of the drilling mudat or near specific depths within the wellbore. Typically, objectcompression that begins at the surface has limited value. In theseapplications, the compressible objects compress from the surface for apredetermined compression interval or range, which extends down to aspecific depth. As a result, these compressible objects may be utilizedfor some specific land drilling applications, but may not be useful indeepwater environments or deeper drilling intervals. To provide a changein the density over a specific predetermined pressure interval forspecific depths or external pressure, the starting depth and depthinterval for the predetermined pressure interval over which thecompression occurs may preferably be adjusted by the compressibleobjects. For example, the initial internal pressure of the compressibleobject may be selected based on the depth at which a transition in thecompressibility is desired. At depths in the mud column (i.e. drillingfluid within the wellbore) for which the pressure is below the initialinternal pressure of the compressible objects, the Young's Modulus ofthe wall material and the differential pressure across the wall materialcontrol the volume change of the compressible objects. At depths forwhich the pressure in the mud column is above the initial internalpressure, the volume change of the compressible objects graduallybecomes dominated by the compressibility of the gas. That is, thepredetermined compression interval is a pressure range from an externalpressure that is about equal to the internal pressure of thecompressible object to an external pressure that substantiallycompresses the compressible object (i.e. compresses the compressibleobject into a compressed state, which is discussed further below). Assuch, compressible objects may be fabricated to begin compression at ornear a specific pressure or depth and/or for a specific predeterminedpressure interval to provide a density change in specific portions orintervals of the wellbore.

To compress at a specific depth, the walls of the compressible objectsmay be designed to maintain a predetermined internal pressure. Theinitial internal pressure of the compressible objects for a givendrilling mud density is determined by the depth at which a transition togas compression is dominated by volume change of the compressibleobjects. Typically, an internal pressure greater than about 200 psi(pounds per square inch) at atmospheric pressure, greater than 500 psiat atmospheric pressure, greater than 1500 psi at atmospheric pressureor more preferably greater than 2000 psi at atmospheric pressure, may beutilized. For a given initial internal pressure, the achievable objectcompression ratio is dependent on the ratio of the wall thickness to theeffective diameter of the compressible object. While the wall thicknessis preferably as thin as possible, the lower limit of the wall thicknessis defined by the minimum thickness capable of containing the desiredinternal gas pressure at an external pressure of about 1 atmosphere,which is typically encountered at the surface 106. Accordingly, amaterial with a tensile strength greater than 10,000 psi may typicallybe utilized, as discussed below, to maintain the internal pressure forthe compressible object. As such, the internal pressure may be in arange from 200 psi up to the tensile strength of the shell material atatmospheric pressure, in a range from 2000 psi to the tensile strengthof the shell material at atmospheric pressure, and/or in a range from1500 psi to 3500 psi at atmospheric pressure.

Further, for a given internal pressure and diameter of a compressibleobject, the minimum wall thickness that may be used is therefore definedby the elastic limit of the tensile strength of the wall material.Within these strength limitations, it is desirable to minimize the wallthickness because the ratio of the volume of the wall material to thetotal volume of the compressible object sets an upper limit on themagnitude of the achievable compression ratio, as noted above.Accordingly, while the compressible object may include a variety ofshapes, such as cubes, pyramids, oblate or prolate spheroids, cylinders,pillows, for example, spherical and elliptical objects with spherical ornear spherical inflated geometries are useful for reasons related to theoptimization of the compressible mud rheology. Accordingly, thecompressible objects may include elliptical and/or spherical objects,such as pressurized hollow metallic spherical and elliptical objects,with an aspect ratio (i.e., the ratio of the major diameter to the minordiameter) of between about 1 and 5 to provide compression ratios of upto 5:1 or greater.

The design of the compressible object may be further complicated bystructural instabilities. For instance, a spherical object for a giveninternal pressure and diameter may be restricted by structuralinstabilities characteristic of the spherical object's architecture. Thestructural instabilities may include local strains, such as equatorialbuckling instability during the inflation phase and the cap bucklinginstability during the compression phase. As such, the design of thecompressible object may also be adjusted to compensate for, or reducethe localized strains and instabilities during expansion and compressionof the compressible objects. Accordingly, the Finite Element Analysis(FEA) modeling of a spherical object, which may be one embodiment of acompressible object, is discussed further below, as shown in FIGS.2A-2D.

FIG. 2A is an exemplary chart and embodiments of a compressible object.In the chart 200, a compressible object is a nearly spherical object,which has an aspect ratio of about 1.0 and wall thickness of 10 microns.The aspect ratio of an object is defined as the ratio of the major axisover the minor axis, which is discussed further below.

In FIG. 2A, the chart 200 of maximum strain 202 versus compression ratio204 of the elastic spherical object is shown. The maximum strain 202 isthe largest strain at any point on the compressible object in thatstate. The chart 200, which is generated from a FEA modeling tool, suchas ABAQUS™ FEA, includes a response curve 206 of the spherical object indifferent states. As indicated by the response curve 206, a linearelastic deformation in excess of about 12% is required to provide acompression ratio of at least 5:1. Along the response curve 206, themaximum elastic deformation does not occur uniformly over the objectsurface during compression, but is localized due to bucklinginstabilities during compression.

Specific examples of the localized strain on the object are shown inFIG. 2B. In FIG. 2B, a partial view of an object 210, such as aspherical or elliptical object, subjected to compression pressure thatis external to the object is shown. The elastic deformation of theobject 210 as it is compressing is dominated by strain localizationassociated with a cap buckling instability, which is indicated by thedepressed region 214. The cap buckling instability is a collapse of thedepressed region 214 due to the inability of the structure to resist theexternal pressure loaded on that region. In particular, the regions 216are the locations or areas of the largest localized strain, which areplotted in the response curve 206 of FIG. 2A. The severity of thisinstability has been shown to increase with increasing wall thickness

Based on the discussion above, the compressible object should have atensile strength sufficient to handle the internal pressure and arecoverable linear elongation or elastic strain large enough to handlethe required deformation. If the spherical or near sphericalcompressible object shell is assumed to be metallic, then the metal ormetal alloy should have sufficient tensile strength within its elasticlimit to contain the internal pressure and at least 12% recoverablelinear elongation. While the tensile strength may be easily achieved,few metals or metal alloys have an elastic strain limit in excess of 1%.If the recoverable linear elongation of greater than 1% is desired,typical materials may not be sufficient. The exceptions to thislimitation are some amorphous metal alloys with a limit of elasticstrain approaching about 2% and the shape memory alloys (e.g., the Nitolfamily of NiTi alloys), which exhibit pseudo-elastic strains of up to 8%with less than about 0.1% permanent deformation. Accordingly, typicalmetal or metal alloys cannot provide the at least 12% recoverable linearelongation if a spherical structure is utilized as the initial shape.

To provide the required recoverable linear elongation, the compressibleobject may be designed to divide the deformation of the compressibleobject into different states. For instance, the compressible objects mayhave three different states, such as an initial state, an expandedstate, and a compressed state. In one embodiment the initial state maybe, for example, an oblate spheroid with an aspect ratio less than 1.0.FIG. 2C shows an oblate spherical object 220 having a major axis 222 anda minor axis 224. As noted above, the aspect ratio of the object 220 inthe initial state is defined as the ratio of the major axis 222 over theminor axis 224. With these states, the required deformation of thecompressible object is divided into two phases. The overall requireddeformation may be divided between an expanded state and a compressedstate. The inflation or first phase involves the expansion of thecompressible object from the initial state to the expanded state, whichmay be limited by the tensile strength of the wall material and/orstructural instabilities of the fully expanded compressible objectcharacteristic of the initial state of the compressible objectarchitecture and the initial internal pressure.

In particular, in FIG. 2D, an oblate spherical object 230 with aninitial 4:1 aspect ratio, a 10 micron wall thickness and an inflatedinternal pressure of 10.9 MPa (mega-pascals) is subjected to internalpressure that expands the oblate spherical object 230. The maximum inthe elastic deformation of the object 230 as it is expanding isdominated by strain localization associated with equatorial wallbuckling, which is indicated by the depressed regions 232 and 234. Theequatorial wall buckling instability is a collapse of the regions 232and 234 due to the contraction of the equatorial belt associated withthe inflation of the oblate spherical object 230. In general it has beenshown that the susceptibility of the compressible object to equatorialbuckling increases as the initial aspect ratio of the compressibleobject increases, the internal pressure increases and the wall thicknessdecreases. In this example, the expanded state may be an equilibriumstate with the outside pressure of one atmosphere and where thecompressible object has a spherical or near spherical shape (i.e. aspectratio of about 1.0).

The second phase may involve the compression of the object from theexpanded state back to about the initial state during which thedeformation due to the initial expansion is nearly fully recovered and asubsequent further compression to the fully compressed state, which mayagain be limited by the elastic strain of the wall material of the fullycompressed object. The compressed state may be, for example, anequilibrium compressed shape based on the hydrostatic compressionexerted on the compressible object at a certain downhole depth.Accordingly, the compressible objects may be designed using these statesto provide a suitable compression ratio that is beneficial for usewithin a wellbore.

FIGS. 3A-3C are exemplary embodiments of a compressible object indifferent states in accordance with aspects of the present techniques.In the embodiments of FIGS. 3A-3C, FEA modeling is utilized todemonstrate the different states of a compressible object, which is anellipsoid in this example. Each of these FIGS. 3A-3C is a partial viewof the compressible object in different states. As shown in FIG. 3A, aelliptical object may be in the initial state 300 and have a major axis302 and a minor axis 304 with the aspect ratio being 4:1. In FIG. 3B,the elliptical object may be in the expanded state 306 and have a majoraxis 308 and minor axis 310 and an aspect ratio less than (i.e. <) 4:1.In FIG. 3C, the elliptical object may be in compressed state 312 andhave a major axis 314 and minor axis of 316 and an aspect ratio greaterthan (i.e. >>) 4:1. Accordingly, the aspect ratio for each of thedifferent states 300, 306 and 312 may differ based on the expansionand/or compression of the elliptical object. Compressible objects havingdifferent initial aspect ratios is discussed further in FIG. 4.

FIG. 4 is an exemplary chart of different initial shaped compressibleobjects in accordance with aspects of the present techniques. FEAmodeling is utilized to generate the chart 400 of the maximum strain 402versus compression ratio 404 for different compressible objects having awall thickness of 15 microns. The chart 400 includes a first responsecurve 406 for a spherical object, a second response curve 407 of anelliptical object having a 2:1 aspect ratio, a third response curve 408of an elliptical object having a 3:1 aspect ratio, a fourth responsecurve 409 of an elliptical object having a 4:1 aspect ratio, which maybe the elliptical object in FIGS. 3A-3C, and a fifth response curve 410of an elliptical object having a 5:1 aspect ratio.

As indicated by the response curves 406-410, the maximum strainincreases and decreases between the various states. For objects with aninitial aspect ratio less than 3:1, the maximum linear elastic strainbehavior for compression ratios less than 3:1 is dominated by capbuckling instabilities described above. For compressible objects with aninitial aspect ratio greater than 3:1, the maximum strain decreases fromthe expanded state to a minimum value at or close to the initial state,which is a global minimum for the strain on the compressible object.Then, the maximum strain increases from the initial state until thefully compressed state is reached. As such, the maximum strain at theinitial state of the compressible objects is near zero as indicated bythe response curves 406-410. This aspect is clearly demonstrated by thefourth response curve 409. Along the response curve 409, the expandedstate is located at the point 416, the initial state is located at thepoint 414 and the compressed state is located at the point 412. Clearly,the initial state of the compressible object has the lowest strain incomparison to the expanded and compressed states. In addition, thiscompressible object has a maximum strain of about 0.085, which is aboutthe value of the maximum recoverable strain for the austenite tomartensite phase transformation of the Nitol family of alloys in theirpseudo-elastic state. That is, the response curve 409 indicates that theelliptical object having a 4:1 initial aspect ratio is a suitablestructure and wall thickness to provide the specified compression ratioof greater than 5:1 with an internal pressure useful for the practice ofthe invention disclosed in International Patent Application PublicationNo. WO 2006/007347. Each of the other response curves 406-408 and 410exceed the maximum recoverable strain of 0.085. Strains above theaustenite to martensite phase transformation completion strain ofapproximately 8% may experience permanent deformation resulting inlimited fatigue life in cyclic deformation.

From this chart 400, the inflation and subsequent compression of thecompressible object is bounded by an equatorial buckling instabilityduring the inflation phase and the cap buckling instability describedearlier during the compression phase. By modeling the inflation andsubsequent compression, the initial architecture of the compressibleobject may be designed to minimize the recoverable elongation for thespecific compression ratio. In particular, for a compressible object ofconstant wall thickness fabricated from a NiTi shape memory alloy withan austenite to martensite phase transformation temperature below about0° C. (Celsius) and a target expanded internal pressure of 1500 psig(pounds per square inch gauge), the initial aspect ratio of thecompressible object before inflation may preferably be between about 3and 4 with a wall thickness between about 15 and 20 microns to avoidexceeding about 8% linear elongation anywhere in the wall of thecompressible object for a compression ratio of up to 8:1. As notedabove, to be useful for the practice of Patent Application No. WO2006/007347, the alloy should be in a pseudo-elastic condition. Ordinaryshape memory alloys with transformation temperatures above about 0° C.are not useful for this application. The requirement of an austenite tomartensite phase transformation temperature below about 0° C. recognizesthat the alloy should remain pseudo-elastic over the entire temperaturerange encountered during operation of the compressible objects in thedrilling mud.

Based on the modeling methods discussed above, compressible objects maybe designed of a certain material and having a specific architecture toprovide specific compression ratios that are within the deformationlimitations of existing materials. With these compression ratios, thecompressible objects may be useful for certain applications, such asdrilling and production operations, which are described above. As anexample, the compressible objects may be useful if they provide arecoverable compression ratio greater than or equal to five times theexpanded state at a specific depth interval of interest. Thecompressible objects may be included in the variable density drillingmud in a volume fraction of up to 40% or 50% to provide a change indrilling mud density representative of typical PPGs and/or FGs. Bychanging the density of the drilling mud by adding up to 50% by volumeof small low-density, compressible objects, which may have a diameter ofabout 1 millimeter (mm), the pressure gradient within the wellbore maybe substantially controlled to reduce the number of casing stringsutilized within the wellbore. In particular for a deep-waterapplication, the number of casing intervals may be reduced substantiallybelow that achievable with dual gradient or multi-gradient systemswithout major modification of existing hardware or equipment. As such,the well cost may be reduced by up to 30 to 50% for certainapplications. Accordingly, the selection of the compressible objects andfabrication of the compressible objects is discussed further below inFIG. 5.

FIG. 5 is an exemplary flow chart of the selection and use of thevariable density drilling mud for the drilling system 100 of FIG. 1 inaccordance with certain aspects of the present techniques. This flowchart, which is referred to by reference numeral 500, may be bestunderstood by concurrently viewing FIGS. 1, 3A-3C and 4. In this flowchart 500, compressible objects and drilling fluid may be selected toformulate a variable density drilling mud for a well. These compressibleobjects may include objects that each have a shell enclosing an interiorregion, and wherein the compressible object has (a) an internal pressure(i) greater than about 200 psi at atmospheric pressure, 500 psi atatmospheric pressure, 1500 psi at atmospheric pressure and/or 2000 psiat atmospheric pressure, and (ii) selected for a predetermined externalpressure, wherein external pressures that exceed the internal pressurereduce the volume of the compressible object; (b) wherein the shellexperiences less strain when the external pressure is about equal to theinternal pressure than when the external pressure is above or below apredetermined compression interval of the compressible object or whereinthe shell is configured to experience less strain when the externalpressure is about equal to the internal pressure than when the externalpressure is greater than the internal pressure or less than the internalpressure; and/or (c) compressible objects having a shell that enclosesan interior region at least partially filled with a foam. Then, thevariable density drilling mud may be utilized to enhance the drillingoperations of the well. This process may enhance the drilling operationsby providing a variable density drilling mud that extends the drillingoperations to further limit or reduce the installation of additionalcasing strings. Accordingly, drilling operations performed in thedescribed manner may reduce inefficiencies from utilizing additionalcasing strings from drilling operations.

The flow chart begins at block 502. At block 504, the FG and PPG for awell may be determined. For example, the FG and PPG may be obtained byreceiving information from the drilling location and/or performingcalculation to estimate the FG and PPG. Then, compressible objects maybe selected to provide specific volumetric changes, as shown in block506. The selection of compressible objects may include operationalconsiderations, such as removal of the compressible objects from thedrilling mud for re-circulation at the surface, limiting potentiallydetrimental effects of the high volume fraction of compressible objectson the rheology of the drilling mud and facilitating the flow of thecompressible objects through the pumps and orifices in the flow path. Assuch, the compressible objects may be sized to have an equivalentdiameter between 0.1 millimeter (mm) and 50 mm, and/or preferablybetween 0.1 mm and 5.0 mm. The equivalent diameter is defined as thediameter of a sphere of equal volume as the fully expanded compressibleobject at atmospheric pressure. Further, the selection of compressibleobjects may include utilizing compressible objects of different sizes orvolumes at the surface of the wellbore and/or different shapes to managethe viscosity increases of the drilling mud. The selection of thecompressible objects is further described in FIG. 6.

At block 508, the drilling fluid may be selected. The drilling fluid,which may include various weighting agents, may be selected to provide aspecific density that may interact with the compressible objects tomaintain the drilling mud density between the FG and PPG, which isdiscussed further below. The compressible objects and the drilling fluidmay be combined in block 510. The combination of the compressibleobjects and the drilling fluid may involve mixing or blending thecompressible objects with the drilling fluid, as described inInternational Patent Application No. PCT/US2007/003691, filed 13 Feb.2007. Further, the compressible objects and the drilling fluid may becombined prior to shipping to the drilling location or shippedindividually with the compressible objects and the drilling fluid beingcombined at the drilling location. It should be noted that thecompressible objects may be shipped in refrigerated vehicles, such astrucks and ships, to reduce risks associated with the release ofinternal pressure within the compressible objects.

At the drilling location, the compressible objects and the drillingfluid, which may be the variable density drilling mud 118 (FIG. 1), maybe utilized in the drilling operations, as shown in block 512. Thedrilling operations may include any process where surface fluids areused to achieve and maintain a desired hydrostatic pressure in awellbore and/or the processes of circulating this fluid to, among otheruses, remove formation cuttings from the wellbore. Once the well isdrilled, the hydrocarbons may be produced in block 514. The productionof hydrocarbons may include completing the wellbore, installing deviceswithin the wellbore along with a production tubing string, obtaining thehydrocarbons from the subsurface reservoir, processing the hydrocarbonsat a surface facility and/or other similar operations. Then, the processends at block 516.

FIG. 6 is an exemplary flow chart of the selection and fabrication ofthe compressible objects discussed in the flow chart of FIG. 5 inaccordance with certain aspects of the present techniques. This flowchart, which is referred to by reference numeral 600, may be bestunderstood by concurrently viewing FIGS. 1, 3A-3C, 4 and 5. In this flowchart 600, a process for selecting compressible objects to maintain thedensity of a drilling mud within the well between the PPG and FG isdescribed. Beneficially, the use of compressible objects in the variabledensity drilling mud may enhance drilling operations by reducing thesize of the wellbore and casing strings, and may provide access togreater depths.

The flow chart begins at block 602. At block 604, the FG and PPG for awell are obtained. The FG and PPG may be obtained by receivinginformation from the drilling location and/or performing calculation toestimate the FG and PPG. Then, a structure for each of the compressibleobjects is selected, as shown in block 606. The selection of thestructure for the compressible objects may include using finite elementanalysis (FEA) methods to match structures and geometries ofcompressible objects to properties of the available materials, asdescribed above. At block 608, wall materials for the compressibleobjects are selected. The selection of wall materials may include metalsand/or metal alloy thin films formed mechanically or by depositionalmethods, polymers with or without micro and/or nanofiber re-enforcementin a polymer matrix to achieve the specific properties of the wallmaterial (e.g., as defined by FEA analysis of the object compression).In addition, wall materials may include ex-foliated inorganic mineral asre-enforcement or as a barrier to gas permeability in a polymer matrix;metal and/or metal alloy thin films formed by depositional methods onpolymer surfaces with or without chemical modification of the polymersurface to form a structural wall or a barrier to gas permeation. Themetal and/or metal alloy thin films may be deposited on polymer sheetprior to forming of the compressible object or on a pre-formedcompressible polymer object. The metal layer may be formed on the insideor outside surface of the compressible objects or incorporated within apolymer wall or polymer laminate of the same or different polymers.

Surface treatments may be selected for the fabrication of thecompressible objects in block 610. The surface treatments may includephysical and/or chemical surface treatments to improve the continuityand adhesion of metal and/or metal alloy films on the surface of thepolymer objects or to enhance the chemical and/or physical compatibilityof the polymer or metallic exterior wall of compressible objects withthe drilling fluid.

Once selected, the compressible objects are fabricated in block 612. Thefabrication of the compressible objects may include variouspolymerizations, depositions, surface treatments and other fabricationprocesses used to form the wall structures of the compressible object.For instance, the fabrication of the wall structures may includeco-axial bubble blowing methods where the polymer is the structuralwall; co-axial bubble blowing methods where the polymer is a templatefor the deposition of a metal or metal alloy structural wall; dispersionpolymerization methods where the polymer is a template for thedeposition of a metal or metal alloy structural wall; and/or interfacialpolymerization methods where the polymer is a template for thedeposition of a metal or metal alloy structural wall. The fabricationmay include the deposition of a continuous metal or metal alloy layer onthe surface of a compressible polymer object in either low or highpressure liquid environments using electro or electro-less platingmethods; the deposition of a continuous metal or metal alloy layer onthe surface of a compressible polymer object in high pressure gasenvironments using ultraviolet chemical vapor deposition (UV-CVD)methods; and/or the deposition of a continuous metal or metal alloylayer on the surface of a compressible hollow object under vacuum usingphysical and/or chemical deposition methods. The vacuum depositionmethods may or may not include reducing the internal pressure inside thecompressible object prior to deposition. This may be accomplished forexample, by first reducing the internal pressure of the compressiblehollow object by cooling the pressurized compressible hollow objectpreferably to a temperature below which the gas inside the compressiblehollow object may condense. Further, fabrications may include molding orforming a flat metalized polymer sheet or film into portions ofcompressible objects and joining the components using mechanical,chemical and/or thermal methods; forming a flat polymer sheet or filminto portions of the compressible object before metallization andjoining the components using mechanical, chemical and/or thermalmethods; deposition of a metal or metal alloy on a polymer sheet with orwithout chemical and/or physical pre-treatment to improve adhesion andcontinuity and subsequent removal of the polymer template from the flatfree standing metal or metal alloy sheet by physical, chemical and/orthermal methods resulting in the formation of a thin metallic sheetsuitable for mechanical forming into components of compressible objectsand subsequently joining the components by mechanical, thermal and/orchemical methods; deposition of a metal or metal alloy on a polymersheet pre-formed into a template for free standing metal or metal alloycomponents of the compressible object and subsequent removal of thepolymer template from the metallic component by chemical, mechanicaland/or thermal methods and subsequently joining the components bymechanical, thermal and/or chemical methods.

At block 614, the compressible objects may be verified or tested. Theverification and testing may include cyclic compression tests to verifythe internal pressure and to quantify the fatigue life of thecompressible objects with or without micro-structural analysis of thestructural wall and the joints if any. Then, the compressible objectsmay be stored, as shown in block 616. The storage of the compressibleobjects may include placing the compressible objects in a storagevessel. The compressible objects may be stored at ambient pressure or ata pressure equal to or higher than the internal pressure of thecompressible objects to facilitate packing of the compressible objectsin the storage vessel. Alternatively, the compressible objects may bestored in a cold environment to reduce the internal pressure inside thecompressible objects. The cold compressible objects may then be storedin a vessel at ambient pressure or at elevated pressure to facilitatepacking of the compressible objects in the storage vessel and shippingthe compressible objects to another location, such as the drillinglocation, for storage or other similar activities. The process ends atblock 618.

Accordingly, based on the discussion above, the selection and use ofthese compressible objects may involve different aspects that affect thedesign of the compressible objects. For instance, the nature of thetransition to gas compression controlled deformation is dependent on themechanical properties of the shell or wall material and the evolution ofthose properties in repeated compression cycles. As such, thecompression of hollow objects results in a different gradient of muddensity above and below the depth defined by the initial internalpressure of the hollow objects. Because the use of compressible objectshaving different initial internal pressures may be beneficial to enhanceor extend drilling operations, changing the volume fraction anddistribution of initial pressures of compressible objects may achievethe desired result of maintaining the effective mud weight between thePPG and FG.

Further, the use of different gases may also influence the design of thecompressible objects. For instance, the hollow object may be filled witha mixture of condensable and non-condensable gases. The addition of acondensable gas allows additional flexibility in tailoring the variationof drilling mud density with depth. At the temperature and pressure ofthe gas/liquid phase boundary, the condensable gas liquefies with anincrease in density and a corresponding decrease in volume. The decreasein internal volume of the hollow object results in a step increase ineffective mud density at the depth and temperature corresponding to thephase transition. An additional benefit of using a gas mixturecontaining a condensable gas is the finite internal volume occupied bythe condensed gas at depths once it has condensed because thecompressibility of the condensed liquid is generally lower than that ofthe non-condensable gas. As a result, the condensed liquid volume may beused to set an upper limit on the deformation experienced by the wall ofthe hollow object. This may be utilized to control the fatigue life ofthe flexible objects as they cycle between the bottom of the wellboreand the surface.

Moreover, the operational use may influence the design of thecompressible objects. In particular, confining the volume change to alarge number of small diameter compressible objects mixed into thedrilling mud allows tailoring of the initial size and/or shape of thecompressible objects to achieve a stable composite mud fluid rheologywithin the vertical mud column of the wellbore. To create a usablevariable density drilling mud, the initial properties of the fluid phasefor a given compressible solid volume fraction is selected to suspendboth the rock cuttings and the compressible objects in the wellboreannulus during non-circulating operations. In addition, the viscosity ofthe composite mud has to be configured to be pumped within the wellboreby mud and rig pumps within acceptable limits. Also, the use ofdifferent sized compressible objects may further enhance the operationaluse. These aspects and others are discussed further below.

Architecture of Compressible Objects

To determine the architecture of the compressible objects, as noted inblock 606 of FIG. 6, a finite element numerical modeling method may beutilized. The finite element numerical modeling method may includeimplicit methods and/or explicit methods. In these methods, the shellwalls or elements may be represented by mesh size and shape tailoredwith higher resolution in regions of interest, such as regions of highstress and/or strain for compressible object construction. The finiteelement numerical model may be used to simulate the entire threedimensional object or a segment of the object related to the threedimensional object by symmetry. Further, the architecture of thecompressible objects may be influenced by various criteria, such as thematerials and use of the compressible objects, which are discussed inthis and other portions of the application.

With regard to the use of the compressible objects, it should be notedthat the architecture of the compressible objects may facilitateperiodic removal of the compressible objects from the re-circulatingdrilling mud. This may facilitate limiting potentially detrimentaleffects of the high volume fraction of compressible objects on therheology of the drilling mud and/or facilitate the flow of thecompressible objects through the equipment, such as pumps, and orificesin the flow path. As such, the compressible objects may includestructures having an equivalent diameter in the range of about 0.1 mm(millimeter) to 5.0 mm. The equivalent diameter is again defined as thediameter of a sphere of equal volume as the fully expanded compressibleobject at an external pressure of one atmosphere. In addition, the shapeof the compressible objects may be adjusted to increase the packingdensity and reduce effects on fluid flow. For instance, a spherical orelliptical object may provide the highest packing density and lowesteffects on the fluid flow within the wellbore in comparison to pillow orrod shaped objects.

Another criterion for the architecture is the wall thickness. As notedabove, the wall thickness should be as thin as possible within theconstraints imposed by structural instabilities and the properties ofexisting materials to maximize the compression limit of the compressibleobject. However, the lower limit of the wall thickness is defined by theminimum thickness able to contain the desired internal gas pressure atan external pressure of about 1 atmosphere typically encountered at thesurface of the Earth.

To determine the optimal geometry of the compressible objects, methodsof finite element numerical modeling may be utilized. Finite elementnumerical modeling is well known by those skilled in the art. Thesemethods may include modeling the walls as shell elements of thecompressible objects or as a mesh object with variable mesh size andshape. Certain regions of interest, such as regions of high stressand/or strain for the compressible object construction, may be tailoredwith higher resolution (i.e., smaller mesh size) to provide moreinformation in these regions. Further, the model may be used to simulatethe entire three dimensional (3D) compressible object, a segment of thecompressible object, or a portion of the compressible object that may berelated to the 3D compressible object structure by symmetry.

As an example, one preferred method of analyzing and optimizing thecombinations of compressible object geometry, compressible objectmaterial properties, internal gas properties, internal pressure andresponse of the compressible object to changes in external temperatureand/or pressure is to construct a finite element model of either theentire compressible object or a portion of the compressible object(i.e., a hemisphere, due to symmetry). By using software, such asABAQUS™ or any other suitable FEA analysis package, a finite elementnumerical model may be constructed for the compressible objects. In thismodel, an explicit method may be used to monitor for contact between theinternal surfaces of the compressible objects during compression. Tominimize oscillations during external pressure modifications, theexternal pressure may initially be set equal to the internal pressure.Then, the external pressure may be slowly decreased down to ambient,which may be done over a period (e.g., 0.5 sec.) sufficient tosubstantially eliminate dynamic artifacts in the simulation. Dependingon the flow behavior of the wall material and any occurrence ofbuckling, the amplitude and rate of external pressurization anddepressurization may be adjusted to minimize oscillations. Once thefinite element numerical model has been constructed, other analysis maybe performed. For instance, the compressible object may undergo apressurization cycle test. Then, an analysis of the data from thepressurization cycle test may be utilized to gain insight on the effectof compressible object geometry, compressible object dimensions and/ormaterial properties. In addition, if the numerical model is constructedusing shell elements, sudden changes in mesh geometry should be avoidedto reduce the potential for anomalies in local stress calculations.

As a specific example, the finite element numerical model of thecompressible object of FIGS. 3A-3C is discussed. In these embodiments,the compressible object has the shape of an oblate ellipsoid. Theinitial aspect ratio may be in the range of 1 to 10, with a morepreferred aspect ratio being in the range of 2 to 5. The use of aninternally pressurized oblate ellipsoid hollow compressible object withan initial aspect ratio greater than 1 has the advantage that at ambientexternal surface pressure, the ellipsoid object inflates and approachesan aspect ratio of about 1 depending on the internal pressure andmaterial properties, as shown in FIG. 3B. If the ellipsoid object has aninitial aspect ratio of 4:1, a uniform NiTi alloy wall thickness of 10microns and an internal pressure of 1500 psig, the aspect ratio in theexpanded state is about 1.22:1. As the external pressure increases, theellipsoid object tends to return to an initial state 300. In the initialstate 300, the aspect ratio of the ellipsoid object is that of theoriginal design with little elastic strain, as shown in FIGS. 3A and 4.Then, as the pressure continues to increase, the ellipsoid object iscompressed further into a compressed state 312, as shown in FIG. 3C.

Wall Material for Compressible Objects

In addition to the architecture, various materials may be utilized forthe wall of the compressible objects based on the criteria discussedabove, as noted in block 608 of FIG. 6. In particular, the shell or wallmaterials may be divided into two classes of commercially availablematerials, which are metal materials and polymer materials. The metalmaterials may include metals, metal alloys, and alloys withpseudo-elastic behavior (e.g., deformations associated with a reversiblestress induced structural phase transformation). Further, thesuper-plastic behavior of ultra thin (i.e., <500 Angstroms (A)) metal ormetal alloy films may also be used to make a wider variety of metals andmetal alloys (e.g., Aluminum (Al), Copper (Cu), Nickel Titanium (NiTi),etc.) suitable for application as a thin permeation barrier inconjunction with a non-metallic load bearing wall that satisfies themechanical properties of the load bearing wall. Specifically, the metalmaterials may include, but are not limited to, binary or near binaryNiTi, ternary alloys of NiTi with iron and chromium alloying additions,Magnesium-40Copper (Mg-40Cu) alloys, Beta-Titanium-9.8Molybdenum-4Niobium-2Vanadium-3Aluminum (β-Ti-9.8Mo-4Nb-2V-3Al) alloys,metallic glasses and amorphous metals (e.g. Zirconium (Zr), Iron (Fe)and/or Magnesium (Mg) based alloys) and the like. The polymericmaterials may include polymers and polymer blends with or withoutreinforcement (e.g., micro to nano-fiber, nanotubes, exfoliatedinorganic fillers with appropriate orientation within the polymer walletc.). Examples of polymers with suitable properties include but are notlimited to commercially available polyimide, such as Ubilex-R andUbilex-S.

Because each of these materials has specific properties, such as tensilestrength and recoverable elongation, the material utilized in the wallsof the compressible objects is a factor in determining the thickness ofthe wall. The determination may be based upon finite element numericalmodeling, as noted above, to evaluate different thicknesses of the shellor wall with different materials. For instance, if the load bearing wallmaterial is a metal or metal alloy, only metals and metal alloys withsufficiently high elastic or pseudo-elastic behavior should be selectedbecause deformations associated with a reversible stress inducedstructural phase transformation have to be recoverable for reuse of thecompressible objects. As noted above, even these selected materials haveto be combined with careful design of the geometry of the exterior shellof the compressible object to avoid strain localization duringcompression and re-expansion. In particular, the geometry and materialmay be utilized for optimization of the wall thickness relative toparticle size; variation of the bearing wall thickness and/or mechanicalproperties with location on the compressible objects' surface; and/orvariation of the aspect ratio and major diameter of an oblate spheroidhollow compressible objects, etc. Accordingly, these various factors areconsidered in selecting a material for the compressible objects.

As an example of the variation of wall thickness, the wall material maybe utilized to influence the compression ratios of the compressibleobject, such as the elliptical object discussed above in FIGS. 3A-3C. InFIG. 7, the FEA calculations provide various shapes that have differentcompression ratios within the limits defined by existing materialsproperties. The FEA calculations may provide compressible objects havingan aspect ratio between 2 to 5, with anequivalent-diameter-to-wall-thickness ratio between 20 and 200, or morepreferably between 50 and 100. As shown in FIG. 7, a chart 700 of theeffect of wall thickness is shown for maximum strain 702 of compressibleobjects against the equivalent diameter to wall thickness ratio 704 forvarious shapes, which are shown by curves 706-711, generated from finiteelement numerical modeling. For sphere-shaped compressible objects,curve 706 has a compression ratio of 3.5, curve 707 has a compressionratio of 3, and curve 708 has a compression ratio of 2. For the ellipseshaped compressible objects, curve 709 has a compression ratio between3.5 and 2, curve 710 has a compression ratio between 3 and 2, and curve711 has a compression ratio of about 2. It is clear from the chart 700that compressible objects having an aspect ratio greater than unity witha thinner wall (i.e., higher equivalent-diameter-to-wall-thicknessratio) are preferable because they provide higher compression ratioswith correspondingly lower maximum strain. Also, it may be preferable tomaintain the maximum strain below a specific value, of about 0.06 asdefined by the maximum allowable strain to achieve adequate fatigue lifeof the structural wall. Typically, a minimum fatigue life of at least2000 to 3000 cycles is desirable. Based on this limitation, an ellipsoidobject with an aspect ratio at 2 or more andequivalent-diameter-to-wall-thickness ratio greater than 65 provides acompressible object that is below the specific value, as shown on curve711.

In addition to being a single material, the walls of the compressibleobjects may include two or more layers. For instance, the layeredcomposite shell may include a load bearing structural layer or wall anda gas permeation barrier wall. The load bearing wall may be a relativelythick wall having a thickness in the range of 1 micron to 50 microns anda gas barrier wall may be a thin wall having a thickness in the range ofless than or equal to 5 microns. For example, the load bearing polymerwall, which may have a hollow interior or be deposited on a polymer foamtemplate, may be utilized to provide the structure of the compressibleobject. The gas barrier wall, which may be internal or external to theload bearing wall may be a metal or metal alloy permeation barrier layerthat contains the internal pressure and has a thickness below 500Angstrom. Alternatively, the compressible objects may have a thin (i.e.,<5 micron) shell wall, which is either hollow or deposited on a polymerfoam, with a relatively thick (i.e., 1 micron<wall thickness<50 microns)load bearing and barrier wall of metal or metal alloy layer thatprovides structural support and a barrier to gas permeation.

Selection of Surface Treatments for Compressible Objects

As discussed in block 610 of FIG. 6, various surface treatments may beutilized for the compressible objects. The surface treatments may beutilized to improve the continuity and adhesion of polymer layers ormetal and/or metal alloy films on the surface of the compressibleobjects, such as polymer objects. Accordingly, the surface treatmentsmay be utilized to enhance specific properties, such as compatibilitywith the base fluid and the permeability of the shell layers to maintainthe internal pressure, which is discussed further below.

For internally pressurized compressible objects having a load bearingwall of a polymer and/or an elastomer with or without reinforcement, asurface treatment may be utilized to enhance the continuity of a metaland/or non-metal film deposited on the surface of the polymer to reducethe gas permeability of the load bearing wall. In general, elastomers,crystalline polymers and/or polymer blends have gas permeabilities toolarge to be useful for the fabrication of the compressible objects.Accordingly, in addition to the incorporation of exfoliated inorganicfillers in the polymer wall, the deposition of a continuous, thin (i.e.,<500 Angstrom) low gas permeability coating either on the surface of thewall or incorporated into a layered wall structure may be used. Forexample, the coating may be a thin metal, metal alloy or inorganic gaspermeation barrier, which is applied through a variety of physicaland/or chemical treatments to the exterior of the surface wall of thecompressible object. In particular, the deposition coating may be lessthan 500 A in thickness and include Al, NiTi, or any other suitablematerial. Surface treatments to enhance the uniformity and/or continuityof these permeation reducing layers may include: (1) Anionicfunctionalization of the surface e.g., sulfonation, carboxylation, i.e.,acid formation, as well as other anionic functionalizaton methodologiesand chemistries used by those well-versed in the state of the art. (2)Cationic quaternization functionalization chemistries e.g., sulfoniumsalts, phosphonium salts, ammonium salts, used by those well-versed inthe state of the art. (3) Zwitterionic ionic functionality andamphoteric functionality practiced by those well-versed in the state ofthe art. (4) Maleation functionalization and the associated reactionsknown by those well-versed in the state of the art. (5) Controlledoxidation e.g., peroxides, high temperature oxygen plasma etching,ozone, and the like. (6) Chemical vapor deposition methodologies andassociated chemistries. (7) Corona discharge approaches to surfacefunctionalization used by those well-versed in the state of the art.

A wide variety of methods are available for deposition of metal and/orinorganic barrier coatings. One of the factors that may influence theselection of deposition method is the internal pressure of thecompressible object. For instance, if little or no initial internal gaspressure is contained within the compressible objects, then a lowpermeability metal, metal alloy or inorganic coating may be utilizedthrough various low pressure physical and chemical deposition methods touniformly coat the non-planar geometry of the compressible objects. Ifthe compressible object's internal pressure and the wall permeability issuch that the low pressure environment (i.e., typically <1×10⁻³ mm ofHg) required for low pressure physical and chemical deposition methodsis not maintainable, deposition methods compatible with the internal gaspressure and relatively high wall gas permeability may be used. In thisexample, the compressible objects may be maintained in a high pressuregas or liquid environment to prevent loss of internal pressure throughthe wall of the compressible object during storage and coating. For ahigh pressure liquid environment, the coating of the wall surface may beaccomplished, for example, by electro or electro-less plating usingmethods familiar to those skilled in the art. For the high pressure gasenvironment, the coating of the wall surface may be accomplished by, forexample, chemical vapor deposition (CVD) or ultraviolet chemical vapordeposition (UV-CVD) deposition.

Alternatively, the internal gas pressure inside the compressible objectsmay be reduced to a level that allows application of a range ofcommercial low pressure physical and chemical deposition methodsavailable for an un-pressurized object or polymer sheet. In thisexample, a gas, which may be condensed by lowering the temperature ofthe compressible object, may be utilized for the internal pressurizationof the compressible object. For instance, if the gas internal to thecompressible object is oxygen (O) at a pressure of 10 mPa, subsequentcooling the compressible objects to the temperature of liquid nitrogen(LN₂) at atmospheric pressure may reduce the internal pressure to lessthan or equal to 1×10⁻³ mm of Hg.

Similar considerations for a hollow polymer load bearing wall may beapplied for internally pressurized compressible objects having a loadbearing wall of polymer and/or elastomer foam and gas barrier wall of ametal and/or non-metal permeation barrier, or for a polymer and/orelastomer ultra thin hollow shell or a polymer and/or elastomer foamused as a template for deposition of a load bearing metal and/or metalalloy wall, as noted above. In the latter example, an ultra thin polymershell or polymer foam may be utilized as a template for the depositionof a relatively thick metal and/or metal alloy load bearing wall. Themetal or metal alloy load bearing wall in this example may have athickness from about 5 microns to 50 microns. The ultra thin polymershell or polymer foam may include any polymer and/or elastomer with orwithout reinforcement and surface treatments to enhance the uniformityand continuity of the metal and/or metal alloy load bearing wall. Inthis example, the thickness of the ultra thin polymer shell and/or themechanical strength of the foam need only be sufficient to maintain thedesired shape of the particle during the deposition process.

Fabrication of Compressible Objects

As discussed in block 612 of FIG. 6, once the structure and wallmaterials are selected for compressible objects, various fabricationtechniques may be utilized to create the compressible objects. Thesefabrication techniques may include various processes, such aspatterning, deposition, thermo-mechanical processing and other similarfabrication processes. The patterning processes, which are processesthat shape material into another form, such as compressible objects, mayinclude chemical etching, mechanical etching and the like. The etchingprocesses are processes that remove material from a base material. Thedeposition processes, which are processes that coat or transfer amaterial onto another material, may include physical vapor deposition,chemical vapor deposition, electrochemical and/or electro-lessdeposition, metallization, sputtering, evaporation, molecular beamepitaxy and the like. The thermo-mechanical processes, which areprocesses that form or change a materials shape and microstructure, mayinclude cold rolling, hot rolling, swaging, drawing, cutting, tempering,solution annealing and the like.

The fabrication of compressible objects may use various techniques thatare combined to provide desirable properties of the compressibleobjects, as described above. The fabrication route of the compressibleobjects may be determined based on certain desirable properties of thecompressible objects. For example, low gas permeability, objectflexibility, mechanical integrity, low cost, relative ease of objectfabrication, commercial availability of materials, and/orenvironmentally acceptable materials properties are some of theproperties that may be considered. Other properties may include,desirable range of compressible object sizes, size distributions, andaspect ratios, potential surface functionalization approaches to enhancepolymer/metal adhesion, ability to incorporate “excess” blowing agent(s)to produce a hollow object containing a high pressure gas interior(e.g., the use blowing agent to internally pressurize hollow objects,fill with high pressure gas and the like) among other features.

Accordingly, the fabrication processes may be configured to createcompressible objects that are gas filled polymer objects includinginternal structures being either hollow or at least partially filledwith foam. For instance, FIGS. 8A-8B are exemplary embodiments offabrication processes that create compressible objects having hollowinteriors. Similarly, FIGS. 9, 10 and 11A-11B are embodiments offabrication processes that create compressible objects having foaminteriors or based upon foam templates.

A. Fabrication of Compressible Objects as Hollow Objects

The fabrication processes described below relate to the fabrication ofcompressible objects that are formed as hollow objects, which may or maynot be gas filled. These fabrication processes may be utilized to formcompressible objects that each has a shell enclosing an interior region,each of the compressible object has (a) an internal pressure (i) greaterthan about 200 psi at atmospheric pressure, 500 psi at atmosphericpressure 1500 psi at atmospheric pressure or 2000 psi at atmosphericpressure and/or having a shell that encloses an interior region and (ii)selected for a predetermined external pressure, wherein externalpressures that exceed the internal pressure reduce the volume of thecompressible object; (b) the shell configured to experience less strainwhen the external pressure is about equal to the internal pressure thanwhen the external pressure is greater than the internal pressure or lessthan the internal pressure or the shell that experiences less strainwhen the external pressure is about equal to the internal pressure thanwhen the external pressure is above or below a predetermined compressioninterval of the compressible object; and/or (c) the shell is at leastpartially filled with a foam. While a variety of fabrication processesare described, FIGS. 8A-8B are exemplary embodiments of fabricationprocesses that create compressible objects having hollow interiors.

FIGS. 8A-8B are exemplary embodiments of fabrication processes utilizedin the flow chart of FIG. 6 in accordance with certain aspects of thepresent techniques. In FIG. 8A, an exemplary embodiment of an apparatusfor creating compressible objects in accordance with the presenttechniques is shown. In this embodiment 800, compressible objects, suchas hollow polymer shells or polymer foam structures, may be fabricatedin a pressurized environment formed by a pressurized chamber 802. Forexemplary purposes, the compressible objects are shown as hollow polymershells 804 with a gas interior 806, but may include polymer foamstructures and other compressible objects discussed above.

In this fabrication process example, a coaxial bubble blowing orifice808 at the end of the center tube 810 is enclosed in a coaxial tube 812in a pressurized chamber 802. Sufficient differential pressure isindependently applied within the annulus formed between the center tube810 and the coaxial tube 812 and within the center tube 810 of theorifice to shape the polymer material 814 into hollow polymer shells 816that are filled with gas 818 from the center tube 810. In this manner, agas 818 filled polymer bubble 820 is formed and subsequently detachesfrom the coaxial bubble blowing orifice 808. The pressurized chamber 802may be filled with gas or liquid or a combination thereof and theseparation in the case of bubble formation may be caused by surfacetension, gravity, buoyancy, fluid flow or any combination thereof. Oncethe polymer bubble 820 detaches, the polymer bubble 820 may be droppedinto a crosslinking bath 822 within a bath vessel 824 that promotescrosslinking of the polymer wall. The chemical nature of thecrosslinking bath may be determined by the specific polymer chosen forthe wall material and well known to those skilled in the art of polymersynthesis. Following the hardening bath, the hollow polymer shells 804with a gas interior 806 is formed and may then be removed by transfer toa pressure interlock chamber (not shown) where the crosslinking fluid isseparated from the pressurized compressible objects and the compressibleobjects are transferred to a container for storage.

Further, during or after polymerization and/or separation of the hollowpolymer shells 804, the pressure surrounding the hollow polymer shells804 may be lowered to expand the hollow polymer shells 804 into itsfinal size and shape in the expanded state. This expanded state may bepredetermined by wall thickness, material mechanical properties, objectarchitecture and internal pressure before, during or after cooling ofthe walls. If the polymer wall is the load bearing member, expansion ofthe diameter following synthesis may be used to alter the mechanicalproperties of the polymer wall. For example, by strain re-orientation ofthe polymer chains and/or re-orientation of the reinforcement in thepolymer wall of the hollow polymer shells 804.

Specific adjustments may be incorporated for the fabrication processbased on the materials utilized. For instance, if the polymer material814 is a polymer melt with or without reinforcement, the orifice 808 maybe heated to reduce the melt viscosity to achieve the desired flowproperties of the polymer melt. Also, if the polymer material 814 is apolymer monomer or a mixture of monomers with or without reinforcementand with or without an initiator, the polymerization of the walls of thepolymer bubble 820 after separation from the orifice 808 may beaccomplished by a variety of processes, such as ultra violetpolymerization, free radical polymerization, thermo-chemicalpolymerization, etc., which are familiar to those skilled in the area ofpolymer synthesis.

In FIG. 8B, another exemplary embodiment 830 of an apparatus forcreating compressible objects in accordance with the present techniquesis shown. In this embodiment 830, compressible objects, such as hollowpolymer shells or polymer foam structures, may be fabricated in apressurized environment formed in a pressurized chamber 832. Thepressurized chamber 832 is divided into a lower chamber 838 having a gasinlet 840 and an upper chamber 842 having a fluid inlet 844 and a fluidoutlet 846. For exemplary purposes, the compressible objects are shownas hollow polymer shells 834 with a gas interior 836, but may includepolymer foam structures and other compressible objects discussed above.

In this fabrication process example, a thin film 848 of a suitablepolymer melt or polymer precursor may be formed on a plate 850perforated by a large number of orifices or holes 852. The size andspacing of the holes 852 may be arranged to cause the continuousformation of gas filled bubbles 854, which have a hollow polymer shell834 with a gas interior 836, that separate and float off the plate 850and into a pressurized fluid filling the upper chamber 842 when theplate 850 is pressurized from below at a desired differential pressurebetween the upper and lower chambers 838 and 842. It should be notedthat a variety of alternative geometries of holes may be utilized toform internally pressurized hollow compressible objects from a thin filmof polymer precursor and/or polymer melt. The gas filled bubbles mayexit the upper chamber 842 through the fluid outlet 846 and may beseparated from the fluid by density difference and subsequentlytransferred to a container for storage.

As an alternative exemplary method for creating compressible objects,metal, metal alloy and/or polymer tubes may be utilized to form thecompressible objects. In this fabrication process, compressible objectsare formed from a tube material by cutting the tube material intodesired lengths and closing the ends of the tube material usingmechanical, chemical or thermal methods. The internal pressure of theresulting compressible objects, which may be formed in the shape of apillow, sphere, oblate spheroid, ellipsoid of revolution or any otherdesirable shape may be controlled by closing the cut ends of the tubeand forming the desired shape in a controlled pressure environment. Thepressurized environment may be a pressurized chamber, which is similarto the pressurized chambers discussed above. In addition, thecompressible objects may be formed either before or after metallizationof the polymer wall of the tube material from a polymer and/or elastomertube with or without reinforcement.

As another alternative example method for creating compressible objects,preformed sheets may be utilized to form the compressible objects. Inthis method, mechanical, thermal or chemical joining of preformed sheetsmay be utilized to fabricate compressible objects. The preformed sheetsmay include a layered composite structure, which may include twoembodiments. One embodiment may be a relatively thick structural loadbearing polymer wall combined with a relatively thin continuous metal,metal alloy and/or non-metal permeation barrier layer. In particular,the structural load bearing polymer wall may have a wall thicknessbetween about 5 micron and 50 microns, while the continuous metal ormetal alloy permeation barrier layer may have a wall thickness that isless than about 500 Angstrom. The second embodiment being a thin polymersheet as a template for the deposition of a relatively thick metal ormetal alloy layer that serves as both a structural wall and a barrier togas permeation. For instance, the thin polymer sheet may be less thanabout 5 micron, while the metal or metal alloy layer may have a wallthickness between about 5 micron and 50 microns. Any combination oflayered or multiply layered embodiments with polymer thickness and metalor metal alloy thickness within these limits may be utilized for otherembodiments.

To fabricate these compressible objects, the one or more layeredpre-formed sheets may be fabricated flat and subsequently molded into apre-formed object component using any of a variety of polymer sheetand/or film forming methods familiar to those practiced in the art.Examples include metalized polymer sheet for food packaging, metalizedMylar sheet for party balloons, decorative metal coatings on polymersfilms and metalized polyimide film for aerospace thermal barriers. Ifthe pre-formed object components are to be joined to form thecompressible objects, the joining of the preformed object components maybe accomplished by a variety of methods familiar to those practiced inthe art of polymer film joining. Examples include but are not limitedto, thermal bonding, adhesive bonding, mechanical joining and the like.

In this exemplary fabrication method, the metal or metal alloy layer maybe formed on the interior or exterior of the compressible object usingthe same range of physical and/or chemical methods described above andknown in the field for deposition of the metal, metal alloy and/ornon-metal coatings. For instance, the metal or metal alloy layer may beapplied to the exterior and/or the interior surface in a manner similarto the methods described for deposition on co-axially blown bubbles orbubbles formed by dispersion polymerization above. The coated polymerwall may then be thermo-mechanically molded into the pre-form to havethe metal or metal alloy layer on the interior surface, the exteriorsurface or both. In this embodiment, the reinforcement, surfacetreatment for improved continuity and adhesion and the reorientation ofthe reinforcement and/or the polymer chains by mechanical stress mayalso apply to the fabrication of the flat preformed sheets and may beperformed in a manner similar to the co-axial blowing or dispersionpolymerization.

As an additional fabrication technique, the method of composite sheetfabrication outlined above may also be used to fabricate free standingrelatively thick metal and metal alloy sheet suitable for mechanicalforming into the components of compressible or collapsible objects orparticles. This approach to the fabrication of free standing metal ormetal alloy sheet is particularly useful when thin metallic sheet isdifficult to fabricate by conventional thermo-mechanical methods used inthe fabrication of metal sheet. In particular, the metal and metal alloysheet may have a thickness between about 5 micron and 50 micron. To forma free standing metallic sheet, the polymer template may be removed fromthe thin metallic sheet following deposition of the metal or metal alloybefore or after any additional thermo-mechanical treatment required toconsolidate the deposited thin sheet. Removal of the polymer templatemay be accomplished by a variety of mechanical, chemical and/or thermalmethods known to those of ordinary skill in the art. Alternatively, thepolymer template sheet may be pre-formed in the components of thecompressible objects prior to deposition of the metal or metal alloythin film to form a free standing metal or metal alloy pre-form.

As another fabrication technique, hollow compressible objects may beformed by physical and/or chemical vapor deposition (as described above)of the chemical constituents of a thermoset polymer onto thermallydepolymerizable hollow polymer template or polymer foam. Subsequent todeposition, the themoset polymer constituents may be partially reactedtogether by raising the temperature to form a self supporting themosetpolymer preform layer on the surface of the depolymerizable hollowpolymer shell or polymer foam template. Subsequent to the formation ofthe self supporting thermoset polymer preform layer, the temperature maybe further increased to depolymerize the hollow and/or foam template andthe depolymerization products removed from the resulting hollow selfsupporting object by diffusion through the thermoset preform wall.Finally, the partially cured self supporting hollow preform thermostobjects may be placed into a high pressure vessel and the pressureinside the hollow objects equilibrated by diffusion through thethermoset preform wall with a high gas pressure established inside thevessel. Subsequently, the temperature may be raised further in the highpressure gas environment to fully cure the thermoset polymer in order tolower the gas permeability of the wall and to achieve the optimummechanical properties of the wall material. As before, metallization ofthe exterior surface of the fully cured and pressurized hollow thermosetpolymer shell may be accomplished by the methods described above for thecoaxially blown pressurized hollow polymer shells.

Further, as another embodiment, the compressible objects may bemechanically conditioned during fabrication to strengthen the structuralwall of the compressible objects by reorientation of the micro and/ornano-fiber reinforcement and/or the polymer chains including the wallmaterial by mechanical stresses. This mechanical conditioning mayinclude, but is not limited to, expansion of the compressible object toits final size and shape.

B. Fabrication of Compressible Objects Using a Foam Template

In addition to the fabrication of hollow objects, fabrication processesmay utilize a foam template to create a specific shape in thefabrication of the compressible objects. These fabrication processes mayform compressible objects having a shell that encloses an internalregion and (a) an internal pressure (i) greater than about 200 psi atatmospheric pressure, 500 psi at atmospheric pressure, 1500 psi atatmospheric pressure or 2000 psi at atmospheric pressure and (ii)selected for a predetermined external pressure, wherein externalpressures that exceed the internal pressure reduce the volume of thecompressible object; (b) the shell at least partially filled with afoam; and/or (c) wherein the shell is configured to experience orexperiences less strain when the external pressure is about equal to theinternal pressure than when the external pressure is greater than theinternal pressure or less than the internal pressure or wherein theshell experiences less strain when the external pressure is about equalto the internal pressure than when the external pressure is above orbelow a predetermined compression interval of the compressible object.The foam template may include homopolymers, polymer blends, copolymers,interpenetrating networks, block copolymers, thermosets, thermoplastics,amorphous polymers, crystalline polymers, chemically crosslinkedcopolymers, thermoplastic elastomers, rubbers, liquid crystal polymers,and the like. The foam template may be formed into differentpredetermined shapes, such as, but not limited to, a sphere, rod,lamella, oblate or prolate spheroids, ellipsoids of revolution and/orany combination of these geometries. Further, the foam templates used inthe fabrication of the compressible objects, such as rods, lamellae andthe like may be structured to internally contain a wide range of porestructure (i.e., closed and/or open pores), pore wall thickness, andpore density. These various constructions may be useful for producinghollow objects spanning a wide range of mechanical performance.

Foam pre-forms may be produced via molding procedures, cuttingprocedures, and coating procedures, which may be similar to techniquesrelated to using foams for forming insulation and/or packaging. Thecutting procedures may involve cutting slabstock foam into variousshapes and sizes. The molding techniques, which may include extrusion,blow molding, compression molding and the like, may involve molding thefoam into a desired intricate shape, which may reduce or eliminatelabor-intensive cutting and waste produced from that technique. Inaddition, molding techniques may produce foams having multiple zones ofhardness and with filler reinforcements. The coating methods describedpreviously may also be applied to coating of the foam pre-form, whichmethods may include electroplating, electroless plating, physical vapordeposition, chemical vapor deposition, ultra-violet chemical vapordeposition, and the like, and may be used to form a relatively thinmetal or metal alloy layer over the foam template. The coating of metalor metal alloy layer in this embodiment is utilized to enhance theimpermeability of the compressible objects, which may include a gas (ormixture of gases) under pressure. Alternatively, the polymer templatemay be used for the deposition of a relatively thick metal and/or metalalloy load bearing wall using a molded or mechanically shaped internallypressurized or un-pressurized polymer foam. The metal load bearing wallmay have a wall thickness of about 5 micron to 50 micron and an internalpressure above about 200 psi at atmospheric pressure or greaterdepending on the desired application.

As a first embodiment, blowing agents may be utilized to form the foamtemplate for the compressible objects. Typically, the use of physicalblowing agents results in closed-cell foam template, which may be formedfrom various materials. For instance, polyurethane (PU), polystyrene(PS) and polyvinyl chloride (PVC) are materials utilized inmanufacturing polymer foams. Typically, PU foams are prepared by in situgeneration of carbon dioxide (CO₂), while PS and PVC foams are preparedusing physical blowing agents like nitrogen (N₂) and CO₂. The use ofphysical blowing agents reduces any contaminating solvents fromhindering the process. The use of CO₂ and N₂ has a number of benefits,such as chemical inertness, non-combustibility, natural occurrence, lowcost, ready availability, environmental acceptability (no ozonedepletion) and low human toxicity.

Each of the polymer foaming techniques that use physical blowing agentsrely on the similar principles. These principles are (1) saturation ofthe polymer with a gaseous penetrant (blowing agent) at high pressure;(2) quenching of the polymer/gas mixture into a super-saturated stageeither by reduced pressure or increased temperature; and (3) nucleationand growth of gas cells dispersed throughout the polymer matrix. Uponquenching of the polymer/gas mixture, the solubility of the gas in thepolymer template decreases, which results in clustering of gas moleculesin the form of nuclei. As gas diffuses into the forming cells, the freeenergy of polymer template is lowered. The cell nucleation processgoverns the cell morphology of the polymer material and properties ofthe polymer material. Also, this process may occur homogeneouslythroughout the material or heterogeneously at high-energy regions, suchas phase boundaries. In the high energy regions, the free energy tonucleate a stable void is less compared to homogeneous nucleation. As aresult, preferential nucleation of voids occurs at the interface.

In semicrystalline polymers, the crystalline domains may serve asheterogeneous nucleation points to generate gas bubbles. In general,cell growth is controlled by the time that the gas has to diffuse intothe cells before the quenching, the temperature of the fabricationprocess, the degree of supersaturation, the rate of gas diffusion intothe cells, the hydrostatic pressure or stress applied to the polymermatrix, the interfacial energy and the visco-elastic properties of thepolymer/gas mixture. The stiffness of the polymer template is typicallycontrolled by the foaming temperature. It should be noted that areduction in average cell size generally increases stiffness. The worknecessary to expand the gas cell has to overcome the additional stressresulting from the increased stiffness. By increasing the saturationpressure, the free energy barrier for the formation of stable nuclei isdecreased and additional nucleation sites are formed due to matrixswelling, free volume changes, and/or the formation of crystallineinterfaces. This results in an increased cell density and consequently adecreased average cell diameter. Semicrystalline polymers exhibitconsiderably higher cell densities than amorphous polymers, which areattributed to the contribution of heterogeneous nucleation at theamorphous/crystalline interfacial regions. Because the gas does notdissolve in crystallites, the nucleation is nonhomogeneous, which makesit difficult to control the cellular structure of semi-crystallinefoams. As a result, polymers with a low crystallinity afford foams withan almost uniform structure. As the crystallinity of the polymer isincreased, less desirable non-uniform foams with irregular cell sizesare obtained.

Because the foaming methods using physical blowing agents is versatile,this technique may be used to fabricate closed-cell polymer foamtemplates for the compressible objects. For instance, amorphous as wellas semi-crystalline polymers may be processed within a range oftemperatures close to the glass transition temperature (Tg) up totemperatures just below the melting point of the material. For exemplarypurposes, a fabrication process for forming foam templates and coatingof the foam templates is discussed below in FIG. 9.

FIG. 9. is an exemplary flow chart for fabricating the compressibleobjects in FIG. 6 that use a foam template in accordance with certainaspects of the present techniques. This flow chart, which is referred toby reference numeral 900, may be best understood by concurrently viewingFIGS. 1 and 6. In this flow chart 900, a process for fabricatingcompressible objects having a foam interior is described.

The flow chart begins at block 902. At block 904, the foam may befabricated. The foam may be formed from the various processes, which arediscussed above. The foam may include polymeric materials, such asmoderate to highly crosslinked elastomers with and withoutreinforcement; such as macro, meso to nano-fibers, nanotubes, exfoliatedinorganic fillers (e.g. clays); and polymeric blends with and withoutreinforcement, such as macro, meso to nano-fibers, nanotubes, exfoliatedinorganic fillers (e.g. clays) and the like. At block 906, the foam maybe formed into foam templates. The foam templates may include thevarious shapes, such as cubes, rectangles, rods, squares and otherregular or irregular shapes, which are discussed above. To form the foamtemplates, the foam or polymeric material may be shaped into differentgeometries and sizes by cutting or other suitable processes. Then, atblock 908, the shaped foam templates may be coated with a material. Thematerial may include a thin metal or non-metal coating to reduce gaspermeability that is applied through any suitable deposition techniqueas discussed above. The coatings may include a wide range ofcompositions including pure metals, metal alloys and/or layers ofdifferent metals or metal alloys either alone or in combination withnon-metallic layers among others. At block 910, the coated foamtemplates may be further treated by surface treatments to enhance theadhesion with and promote the continuity of these coatings with thesurface of the polymer foam template. These surface treatments may besimilar to the surface treatments discussed above. The process ends atblock 912.

The coating of these different shaped foam templates is shown in FIG.10. In FIG. 10, various foam templates, such as a pillow object 1002, anelliptical object 1003 and a spherical object 1004 are shown. These foamtemplate objects 1002-1004 are formed into various shapes as discussedin block 906. Then, the foam template objects 1002-1004 may be coated bya metal layer 1006, as discussed in block 908. In particular, the foamtemplate objects 1002-1004 may be coated with a thin metal coating(e.g., copper) through an electroless plating technique. Once coated,the foam template objects 1002-1004 may be further coated by a surfacetreatment layer 1008, as discussed in block 910.

As a specific example of this fabrication process, a first foam templateand a second foam template are described. The first foam template may bean air filled foam microcapsule having cells of about 1000 μm(micro-meter) to 1500 μm in diameter, while the second foam template maybe an air filled foam microcapsule having cells of about 250 μm to 500μm in diameter. These foam templates may be cut into differentgeometries and sizes, as noted above. Then, the shaped foam templatesmay be subsequently coated with a thin metal coating (e.g., copper)through an electroless plating technique. The metal coatings may includea wide range of compositions including pure metals, blends of metals,alloys, shaped memory alloys among others.

Further, it should be noted that the surface treatments may be adjustedfor different foam templates. For instance, if polystyrene is the foamtemplate, it is highly non-polar and chemically reactive polymer. Thedegree of functionalization, i.e., sulfonation, may be controlled via anumber of parameters such as: solvent, sulfuric acid concentration,reaction temperature, reaction time, catalyst, and catalystconcentration. As such, it should be noted that the surfacefunctionalization chemistry and subsequent procedures may be modified toaccommodate the surface chemistry and structure of the material, such asnylon, polyesters, polyurethanes among many other polymeric materials.The surface functionalization and etching may include acid treatment,base treatment, oxidation, nitration, sulfonation, phosphonation amongmany other chemistries. See J. March, “Advanced Organic Chemistry:Reactions, Mechanisms, and Structure”, Third Ed., John Wiley & Sons, NewYork (1985), sections relating to sulfonation, mild oxidation,esterification, carboxylation, free radical addition reactions, freeradical graphing reactions, and quaternization, and the like.

As a first specific example, foam templates may be coated uniformly by aprocess, such as electroless copper plating, to form the rod-like foamedobject. The foam template may be an air-filled foam microcapsule havingcells of about 1000 μm (micro-meter) to 1500 μm in diameter. If thisfoam template is polystyrene, the fabrication process may includefunctionalization of the polystyrene rod by exposure to a 30% solutionof H₂SO₄ for a period of 21 hours. The surface of the functionalizedpolymer can be activated using a tin-palladium (Sn—Pd) activationprocess, otherwise known as seeding. This seeding process is familiar tothose skilled in the art. The process involves successive immersions ofthe polystyrene rod in acidic tin-chloride (SnCl₂) (0.01M) followed byacidic palladium-chloride (PdCl₂) (0.01M) solution with rinsing indistilled water between the baths. A 0.01M Hydrogen-Chloride (HCl) isused after the PdCl₂ to remove the remaining Sn compounds from thesurface. Each of the baths are performed at room temperature. See B.Ceylan Akis, “Preparation of Pd—Ag/PSS Composite Membranes for HydrogenSeparations”, A Thesis, Worcester Polytechnic Institute, (May 2004). Thefunctionalized, Pd seeded polystyrene rod can be placed in a bathflowing at the rate of 73 cc/min (cubic centimeters/minute) containing acopper (Cu) plating solution of CuSO₄.5H₂O, ethylenediaminetetraaceticacid disodium salt dihydrate, NaOH, ethylenediamine, and triethanolamineactivated with formic acid. See Y. Lin and S. Yen, Applied SurfaceScience, 178, 116 (2001); W. Lin, H. Chang, Surface and CoatingsTechnology, 107, 48 (1998); Shu et. al., Ind. Eng. Chem. Res. 36, 1632(1997); Hanna et al. Materials Letters, 58, 104 (2003). Cu can be platedonto the functionalized, Pd seeded polystyrene rod at 40° C. over aperiod of 90 minutes followed by a distilled water wash. The majority ofthe surface can be coated with Cu having a thickness that ranges from0.3-0.6 μm.

Alternatively, if the foam template is an air-filled foam microcapsulehaving cells of about 250 μm to 500 μm in diameter and a sphericalshape, the fabrication process may include functionalization and Pdseeding of the polystyrene sphere, as described above. Using the same Cuplating solution and flow rate, the functionalized Pd seeded polystyrenesphere can be plated at 40° C. for a period of 10 minutes followed by adistilled water wash. As a result, the surface can be coated with a0.1-0.2 μm thick Cu film that follows the contours of the foam surface.

As another example, the fabrication process for a solid Nylon 6/6 ballhaving the diameter of ⅛ inch may include functionalizing and Pd seedingthe solid ball as described above using 0.01M HCl for 10 minutes for thefunctionalization process. Also, the Nylon ball can be reacted in theflowing solution at 40° C. for 4 hours 5 minutes followed by a distilledwater wash, which may be the same Cu plating solution with activatordiscussed above. The resulting Cu plated film can be 10-25 μm thick overthe Nylon ball.

As another exemplary fabrication technique, a hollow gas-filled metallicshell may be fabricated by utilizing the Fraunhofer method for producinghollow metallic objects, as shown in FIG. 11A. See, for example, O.Andersen, U. Waag, L. Schneider, G. Stephani, B. Kieback, “NovelMetallic Hollow Sphere Structures”, Advanced Engineering Materials 2000,vol 2, (April 2000), pp. 192-195. In this embodiment 1100, foamtemplates 1102, which may be Styrofoam templates or any of the polymerfoam templates described above, may be coated with a metallic material1104, which may comprise a metal or metal alloy powder and binder. Thecoating of the foam templates 1102 by metallic material and binder 1104may be accomplished by fluidized bed coating methods in a vessel 1106.The resulting polymer foam templates coated with a metal or metal alloypowder and binder layer 1108 may then be subjected to a furnace 1110 forannealing. In the furnace, the polymer foam template may be thermallydecomposed or reacted to volatile reaction products which are removed bydiffusion through the partially sintered metal or metal alloy wall.Subsequently, the temperature may be raised to drive off the remainingbinder and the metal material is sintered to obtain a dense metal ormetal alloy shell. The resulting compressible objects 1112 may beutilized as part of the variable density drilling mud once it hascooled.

An alternative fabrication method is described in FIG. 11B. In FIG. 11B,either regular or irregularly-shaped metal or metal alloy hollow objectsmay be fabricated by forming a metal or metal alloy layer such as anickel layer on a foam template by deposition from the gas phase onto adisposable foam template. In this embodiment 1120, a foam template 1122,which may be closed-cell polymer foam template, is provided. The foamtemplate 1122 is coated with pigment 1124, such as carbon black or otherpigments that absorb infrared radiation, to form a coated foam template1126. The coated foam template 1126 is then placed into a vessel that isfilled with a gas 1128, such as nickel carbonyl gas. The coated foamtemplate 1126 is then subjected to infrared radiation 1130, which heatsthe coated surface of the coated foam template 1126. As a result of theinfrared radiation 1130, a coating of carbonyl decomposes at the surfaceof the coated foam template 1126 to form a metallic coating 1132, suchas nickel over the foam template 1134. The metallic coated foam template1134 is then sintered in a furnace 1136 at a temperature high enough tomake the foam template decompose and the decomposition products areremoved by diffusion through the metal layer during the sinteringprocess. As a result, a compressible object 1138 is formed with a hollowinterior.

Modification to the Compressible Objects to Address Localized Strain

As an additional embodiment, the architecture of the compressibleobjects may be modified to distribute the localized strain experiencedin the expanded and compressed states. For instance, FEA modelingdemonstrates in the case of ellipsoids of revolution discussed above,that the severity of the cap buckling instability increases as the wallthickness increases and the initial aspect ratio decreases, while theseverity of the equatorial buckling instability increases as the wallthickness decreases and the aspect ratio increases. To expand the designwindow of the compressible object architecture, the wall thickness ofthe compressible object may be varied with the wall thinner at the polesand thicker at the equator. This adjustment of the wall thickness mayprovide support in each of the embodiments to address the localizationof strain in the different regions of the compressible objects. Thevariation of the wall thickness from the pole to the equator may beperformed in a manner that is consistent with certain fabricationtechniques, which are discussed above.

Alternatively, one or more structural members, such as a flange, may beadded to the compressible objects. These structural members, such as aflange, may reduce localized strain for the shell of the compressibleobject. For instance, if the structural member is a flange, it may beadded to the equator of the compressible object to support theequatorial belt against buckling. This flange may distribute thedeformation force along the equator of the compressible object to spreadthe strain from a localized area. For instance, as shown in FIGS.12A-12C, the effect of adding a flange 1202 to a 10 micron wallthickness elliptical object is shown in various states. In this example,the elliptical object may have an inflated internal pressure of 1500psig in this example and formed from a pseudo-elastic material of shapememory alloy, such as NiTi alloy with an austenite to martensitetransformation temperature about 0° C. In FIG. 12A, the compressibleobject, which is an elliptical compressible object having a flange 1202in the initial state 1200. The elliptical object is shown in theexpanded state 1204 in FIG. 12B and the compressed state 1206 in FIG.12C. As shown in the FIGS. 12A-12C, the flange 1202 distributes thelocalized strain to lower the maximum strain experienced by theelliptical object. The benefits from the addition of the flange arediscussed further in FIG. 13.

FIG. 13 is an exemplary chart relating to the addition of a flange tothe compressible object in accordance with certain aspects of thepresent techniques. In FIG. 13, FEA modeling is utilized to generate achart 1300 of the maximum strain 1302 versus compression ratio 1304 fora first compressible object having a flange and a second compressibleobject with no flange. The chart 1300 includes a first response curve1306 for the first compressible object having a wall thickness of 10microns and a flange width of 125 microns, which may be the ellipticalobject of FIGS. 12A-12C, and a second response curve 1308 for the secondcompressible object having a wall thickness of 10 microns with noflange. In the chart 1300, the line 1310 indicates the approximatemaximum recoverable strain for the NiTi alloy and the line 1312 theapproximate maximum allowable strain required to achieve the desiredfatigue life of the object which is discussed above.

As shown in the chart 1300, the addition of the flange reduces themaximum strain experienced by elliptical objects having the samestructure and wall thickness. As such, the equatorial flange may beutilized to expand the design window for compressible objects, which isbelow the permanent deformation limits.

The addition of the equatorial flange may be performed in a manner thatis consistent with certain fabrication techniques, which are discussedabove. As an example, the fabrication of the compressible objects from ametal alloy sheet and subsequent joining at the equatorial flange may beadjusted to provide a flange of a specific width by modifying existingfabrication processes.

Use of Weighting Agents and Other Fluids to Achieve the DeterminedVariable Density Drilling Mud

As noted above, the variable density drilling mud 118 (FIG. 1) mayinclude compressible objects along with the drilling fluid. Theselection of drilling fluid may involve choosing the primary liquidphase component from a number of available fluids. These fluids includewater, oil or combinations of water and oil. The liquid phase is chosenafter considering several factors including cost, compatibility withsubterranean formations, environmental impact and the like. Weightingagents are added to adjust the drilling fluid density. Viscosifiers areadded to provide suspension of the weighting agents and drilledformation cuttings. Other additives provide filtration control toprevent liquid phase migration into the formation or help emulsify freewater into an oil phase.

To compensate for the compressible objects, drilling fluids may includeweighting agents and other fluid to manage the density of the variabledensity drilling mud within the wellbore. The weighting agents mayinclude barite (barium sulfate), hematite (ferric oxide), galena (leadsulfide) and other suitable materials, while the other blending agentsmay include formates, such as sodium, potassium and cesium, and othersuitable materials.

The weighting agents are added to the drilling fluids to increase thedrilling fluid density to be greater than that of the aqueous (water) ornon-aqueous (oil or synthetic) base fluids. For instance, the weightingagents may include barite (barium sulfate), hematite (ferric oxide),galena (lead sulfide) and other suitable materials. These weightingagents are utilized to achieve the desired composite mud density profilefrom surface to target depth (TD). Because the pressure within thewellbore generally increases with depth, the low density compressibleobjects, such as compressible objects, are in an uncompressed state nearthe surface and in the compressed state toward the bottom of thewellbore. When the compressible objects are in the compressed state fromthe downhole pressures, the composite density of the variable densitydrilling mud may be maintained to prevent fluid influxes from theformation and limited to not exceed the formation fracture gradient.When the compressible objects are in the uncompressed state at shallowerdepths, the formation may be exposed to the variable density drillingmud with the rock layers not being as strong and the formation fluidpressure being typically lower. As such, uncompressed state of thecompressible objects may be utilized to lower mud density of thevariable density drilling mud. Accordingly, the various weighting agentsmay be utilized in the drilling fluid to increase the density in theshallower sections of the wellbore to compensate for the expansion ofthe compressible objects.

For example, barite (barium sulfate) may be used to increase the densityof the variable density drilling mud 118. The advantage to using bariteas a weighting agent in drilling fluid is the low cost and highavailability of this material. Barite has a density in the purest formof 4.5 g/cc (gram/cubic centimeter) with drilling grade barite being atleast 4.2 g/cc to carry the American Petroleum Institute brand. Toprovide high drilling mud densities, a large concentration of barite mudmay be suspended in the drilling fluid. For instance, drilling fluidwith a density of up to 19 ppg (pounds per gallon) (2.3 g/cc) maycontain approximately 40% by volume barite. As the volume percentage ofsolids increases, the viscosity of the drilling fluid, particularly athigh shear rates, becomes very high and frictional pressure drop throughthe circulating or wellbore system becomes very high. Accordingly, thedrilling fluid with barite may be combined with the compressible objectswith up to 40% by volume at surface conditions. The result of thiscombination provides higher viscosities where the compressible objectsare uncompressed (at the surface and at shallow depths).

Similar densities of variable density drilling mud may be achieved withlower volume % weighting material by using material with higher density,such as hematite (ferric oxide) or galena (lead sulfide). Hematite has aminimum API density of 5.05 g/cc and may increase drilling fluid densitywith a lower total solids concentration than barite. However, drillingfluids with hematite may be more abrasive than drilling fluids withbarite, which may lead to premature damage or wear to equipment, such asmud pumps, surface equipment, drill string piping and downhole tools(i.e. motors), logging and measurement equipment, for example. Galena(lead sulfide) has a density of 7.5 g/cc and may be used to achieve highdensity with about 40% less solids volume than barite. Galena is arelatively soft mineral and does not prematurely wear equipment.

In an alternative embodiment, blending agents may be utilized with thecompressible objects instead of or in addition to the weighting agents.These blending agents may include formates, such as sodium, potassiumand cesium. For example, a solution of cesium formate in water may yielda solids-free (weighting agent-free) density of about 2.4 g/cc. Thedensity of the cesium formate solution is nearly equal to that oftypical rock or rock cuttings. As a result, the rock cuttings do nottend to settle in drilling fluid with this blending agent. When thecesium formate solution is blended with compressible objects, thevariable density drilling mud may provide high density at high pressureswhere the compressible objects are in the compressed state (i.e. deep inthe wellbore). However, at shallower depths where the compressibleobjects are in the expanded state, the density of the variable densitydrilling fluid is reduced. With this fluid, the increased volume % ofexpanded compressible objects naturally increases the bulk viscosity andassists in the transport of rock cuttings.

Additional viscosity may be provided through the addition ofviscosifying agents, such as naturally occurring bentonite clay orsynthetic polymers, to reduce the rate at which the cuttings andcompressible objects tend to settle due to density differences betweenthe cuttings/compressible objects and the drilling fluid. These types ofviscosifiers aid cuttings transport, while the drilling fluid iscirculating and promote gelation of the drilling fluid when flow isceased thus reducing the cuttings settling velocity and the compressibleobjects settling velocity. The compressible objects may tend to rise orfall within the drilling fluid depending on their state of compression,and compressible object density within the wellbore. At externalpressures less than that required to compress the objects or particles,the compressible objects generally have a lower density than thedrilling fluid. Here the compressible objects tend to rise within thefluid unless the viscosity is sufficient to prevent movement. Whenexternal pressures are high enough to provide sufficient objectcompression, the compressible object density may approach or exceed thatof the drilling fluid. In this environment, the compressible objects maynot move relative to the fluid or may even tend to fall within the fluidunless the viscosity is sufficient to prevent movement.

While the present techniques of the invention may be susceptible tovarious modifications and alternative forms, the exemplary embodimentsdiscussed above have been shown by way of example. However, it shouldagain be understood that the invention is not intended to be limited tothe particular embodiments disclosed herein. Indeed, the presenttechniques of the invention are to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the invention asdefined by the following appended claims.

1. A compressible object comprising a shell that encloses an interiorregion, wherein the compressible object has an internal pressure (i)greater than about 200 pounds per square inch at atmospheric pressureand (ii) selected for a predetermined external pressure, whereinexternal pressures that exceed the internal pressure reduce the volumeof the compressible object and wherein the shell has an engineeredarchitecture designed to compensate for localized strains on thecompressible object during expansion and compression of the compressibleobject.
 2. The compressible object of claim 1 wherein compression of gaswithin the shell dominates the compression of the compressible objectwhen the external pressure exceeds the internal pressure.
 3. Thecompressible object of claim 1 wherein the compressible object has aninternal pressure above about 500 pounds per square inch at atmosphericpressure.
 4. The compressible object of claim 1 wherein the shellexperiences less strain when external pressure is about equal to theinternal pressure than when the external pressure is greater than theinternal pressure or less than the internal pressure.
 5. Thecompressible object of claim 1 wherein the shell has one or morestructural members to reduce localized strain.
 6. The compressibleobject of claim 5 wherein the one or more structural members comprise aflange.
 7. The compressible object of claim 1 wherein wall thickness ofthe shell is varied over the surface of the compressible object toreduce localized strain.
 8. The compressible object of claim 1 whereinwall thickness of the shell is thicker at the equator of thecompressible object to reduce localized strain.
 9. The compressibleobject of claim 1 wherein the compressible object has an internalpressure above about 2000 pounds per square inch at atmosphericpressure.
 10. The compressible object of claim 1 wherein thecompressible object is an ellipsoid object having an aspect ratiobetween 2 and 5 when the external pressure is about equal to theinternal pressure.
 11. The compressible object of claim 1 wherein thecompressible object is an ellipsoid object having an aspect ratiobetween 3 and 4 when the external pressure is about equal to theinternal pressure.
 12. The compressible object of claim 1 wherein theshell has an equivalent-diameter-to-wall-thickness ratio in a range from20 to
 200. 13. The compressible object of claim 1 wherein the shell hasan equivalent-diameter-to-wall-thickness ratio in a range from 50 and100.
 14. The compressible object of claim 1 wherein the shell comprisesex-foliated inorganic mineral as re-enforcement or as a barrier to gaspermeability in a polymer matrix.
 15. The compressible object of claim14 wherein the shell comprises nanofiber reinforcement in the polymermatrix to achieve specific properties for the wall material.
 16. Thecompressible object of claim 1 wherein the shell comprises a gaspermeation barrier layer and a structural layer.
 17. The compressibleobject of claim 16 wherein the gas permeation barrier layer comprises ametal or metal alloy layer and the structural layer comprises a polymerlayer.
 18. The compressible object of claim 16 wherein the gaspermeation barrier layer is formed external to the structural layer. 19.The compressible object of claim 16 wherein the gas permeation barrierlayer is formed internal to the structural layer.
 20. The compressibleobject of claim 1 wherein the equivalent diameter of the compressibleobject is in a range between 0.1 millimeter and 50 millimeter when theexternal pressure is less than the internal pressure.
 21. Thecompressible object of claim 1 wherein the equivalent diameter of thecompressible object is in a range between 0.1 millimeter and 5.0millimeter when the external pressure is less than the internalpressure.
 22. A drilling mud comprising: compressible objects, whereineach of at least a portion of the compressible objects has an internalpressure (i) greater than about 200 pounds per square inch atatmospheric pressure and (ii) selected for a predetermined pressure,wherein external pressures that exceed the internal pressure reduce thevolume of the compressible object and wherein the shell has anengineered architecture designed to compensate for localized strains onthe compressible object during expansion and compression of thecompressible object; and a drilling fluid, wherein the density of thedrilling mud changes due to the volume change of the compressibleobjects in response to pressure changes as the drilling fluid and thecompressible objects circulate toward the surface of a wellbore.
 23. Thedrilling mud of claim 22 wherein compression of gas within thecompressible object dominates the compression of the compressible objectwhen the external pressure exceeds the internal pressure.
 24. Thedrilling mud of claim 22 wherein each of at least a portion thecompressible objects has an internal pressure above about 500 pounds persquare inch at atmospheric pressure.
 25. The drilling mud of claim 22wherein the compressible objects comprises a first portion of thecompressible objects having a first internal pressure and a secondportion of the compressible objects having a second internal pressure,wherein the second internal pressure is different from the firstinternal pressure.
 26. The drilling mud of claim 25 further comprising athird portion of the compressible objects having a third internalpressure, wherein the third internal pressure is different from thefirst internal pressure and the second internal pressure.
 27. Thedrilling mud of claim 22 wherein the equivalent diameter of thecompressible object is in a range between 0.1 millimeter and 50millimeter.
 28. The drilling mud of claim 22 wherein the equivalentdiameter of the compressible object is in a range between 0.1 millimeterand 5.0 millimeter.
 29. The drilling mud of claim 22 wherein thecompressible objects comprises a first portion of the compressibleobjects having a first volume at atmospheric pressure and a secondportion of the compressible objects having a second volume atatmospheric pressure, wherein the second volume is different from thefirst volume.
 30. The drilling mud of claim 29 further comprising athird portion of the compressible objects having a third volume atatmospheric pressure, wherein the third volume is different from thesecond volume and the first volume.
 31. The drilling mud of claim 29wherein the compressible objects comprises a first portion of thecompressible objects having a first shape and a second portion of thecompressible objects having a second shape, wherein the second shape isdifferent from the first shape.
 32. The drilling mud of claim 22 whereineach of the compressible objects has a shell, wherein the shell isconfigured to experience less strain when the external pressure is aboutequal to the internal pressure than when the external pressure isgreater than the internal pressure or less than the internal pressure.33. The drilling mud of claim 22 wherein each of the compressibleobjects is designed to compensate for the localized strains andinstabilities of the compressible object during expansion andcompression of the compressible object.
 34. The drilling mud of claim 22wherein each of the compressible objects has one or more structuralmembers to reduce localized strain.
 35. The drilling mud of claim 34wherein the one or more structural members comprise a flange.
 36. Thedrilling mud of claim 22 wherein each of the compressible objects has ashell, wherein the wall thickness of the shell is varied over thesurface of the shell to reduce localized strain.
 37. The drilling mud ofclaim 22 wherein each of the compressible objects has a shell, whereinthe wall thickness of the shell is thicker at the equator of thecompressible object to reduce localized strain.
 38. The drilling mud ofclaim 22 wherein each of the compressible objects has an internalpressure above about 2000 pounds per square inch at atmosphericpressure.
 39. The drilling mud of claim 22 wherein each of thecompressible objects is an ellipsoid object having an aspect ratiobetween 2 and 5 when the external pressure is about equal to theinternal pressure.
 40. The drilling mud of claim 22 wherein each of thecompressible objects is an ellipsoid object having an aspect ratiobetween 3 and 4 when the external pressure is about equal to theinternal pressure.
 41. The drilling mud of claim 22 wherein each of thecompressible objects has a shell, wherein the shell has anequivalent-diameter-to-wall-thickness ratio in a range from 20 to 200.42. The drilling mud of claim 22 wherein each of the compressibleobjects has a shell, wherein the shell has anequivalent-diameter-to-wall-thickness ratio in a range from 50 and 100.43. The drilling mud of claim 22 wherein each of the compressibleobjects has a shell, wherein the shell comprises ex-foliated inorganicmineral as re-enforcement or as a barrier to gas permeability in apolymer matrix.
 44. The drilling mud of claim 43 wherein the shellcomprises nanofiber re-enforcement in the polymer matrix to achievespecific properties of the wall material.
 45. The drilling mud of claim22 wherein each of the compressible objects has a shell, wherein theshell comprises a gas permeation barrier layer and structural layer. 46.The drilling mud of claim 45 wherein the gas permeation barrier layercomprises a metal or metal alloy layer and the structural layercomprises a polymer layer.
 47. The drilling mud of claim 45 wherein thegas permeation barrier layer is formed external to the structural layer.48. The drilling mud of claim 45 wherein the gas permeation barrierlayer is formed internal to the structural layer.
 49. The drilling mudof claim 22 further comprising weighting agents to control the densityof the drilling fluid and the plurality of compressible objects.
 50. Thedrilling mud of claim 49 wherein the weighting agents comprise one ofbarite, hematite, galena and any combination thereof.
 51. The drillingmud of claim 22 further comprising formates to control the density ofthe drilling mud in mud systems and reduce the addition of insolubleweighting agents that tend to raise viscosity of the drilling fluid andthe compressible objects.
 52. A method associated with drilling a wellcomprising: selecting compressible objects, wherein each of at least aportion of the compressible objects has an internal pressure (i) greaterthan about 200 pounds per square inch at atmospheric pressure and (ii)selected for a predetermined pressure, wherein external pressures thatexceed the internal pressure reduce the volume of the compressibleobject; and wherein the shell has an engineered architecture designed tocompensate for localized strains on the compressible object duringexpansion and compression of the compressible object; selecting adrilling fluid; introducing the compressible objects to the drillingfluid to form a variable density drilling mud, wherein the variabledensity drilling mud provides a density between a pore pressure gradientand a fracture pressure gradient for at least one interval of a well asthe variable density drilling mud circulates toward the surface of thewell; and drilling a wellbore with the variable density drilling mud atthe location of the well.
 53. The method of claim 52 wherein thecompressible objects have an internal pressure above 500 pounds persquare inch at atmospheric pressure.
 54. The method of claim 52 whereineach of the compressible objects has a shell, the shell is configured toexperience less strain when the external pressure is about equal to theinternal pressure than when the external pressure is greater than theinternal pressure or less than the internal pressure.
 55. The method ofclaim 52 wherein each of the compressible objects comprises a pluralityof states, the plurality of states comprising a first state atatmospheric pressure outside a wellbore, and a second and third statewithin the wellbore; wherein the first state has a first volume, thesecond state has a second volume, and the third state has a thirdvolume, wherein the third volume is less than the first volume andgreater than the second volume, and wherein walls of the compressibleobject are under less strain in the third state than in the first stateand the second state.
 56. The method of claim 52 further comprisingblending weighting agents into the drilling fluid to control the densityof the drilling fluid and compressible objects.
 57. The method of claim52 wherein the compressible object has an internal pressure above about1500 pounds per square inch at atmospheric pressure.
 58. The method ofclaim 52 wherein each of the compressible objects has one or morestructural members to reduce localized strain.
 59. The method of claim58 wherein the one or more structural members comprises a flange. 60.The method of claim 52 wherein the compressible object is an ellipsoidobject having an aspect ratio between 2 and 5 when the external pressureis about equal to the internal pressure.
 61. The method of claim 52wherein the compressible object is an ellipsoid object having an aspectratio between 3 and 4 when the external pressure is about equal to theinternal pressure.
 62. The method of claim 52 wherein each of thecompressible objects has a shell, the shell has anequivalent-diameter-to-wall-thickness ratio in a range from 20 to 200.63. The method of claim 52 wherein each of the compressible objects hasa shell, the shell has an equivalent-diameter-to-wall-thickness ratio ina range from 50 and
 100. 64. The method of claim 52 wherein each of thecompressible objects has a shell, the shell comprises a gas permeationbarrier layer and structural layer.
 65. The method of claim 64 whereinthe gas permeation barrier layer comprises a metal or metal alloy layerand the structural layer comprises a polymer layer.
 66. The method ofclaim 52 further comprising combining weighting agents to the drillingfluid, wherein the weighting agents comprise one of barite, hematite,galena and any combination thereof.
 67. The method of claim 66 furthercomprising combining formates with the drilling fluid to control thedensity of the drilling fluid and compressible objects in mud systemsand minimize the addition of insoluble weighting agents that tend toraise viscosity of the drilling fluid and compressible objects.
 68. Amethod associated with the production of hydrocarbons comprising:selecting compressible objects, wherein each of at least a portion ofthe compressible objects has an internal pressure (i) greater than about200 pounds per square inch at atmospheric pressure and (ii) selected fora predetermined well pressure, wherein external pressures that exceedthe internal pressure reduce the volume of the compressible object; andwherein the shell has an engineered architecture designed to compensatefor localized strains on the compressible object during expansion andcompression of the compressible object; selecting a drilling fluid;introducing the compressible objects to the drilling fluid to form avariable density drilling mud, wherein the variable density drilling mudprovides a density between a pore pressure gradient and a fracturepressure gradient as the variable density drilling mud circulates towardthe surface of the well; drilling a wellbore with the variable densitydrilling mud; and producing hydrocarbons from the wellbore.
 69. A methodfor forming a variable density drilling mud comprising: selectingcompressible objects, wherein each of at least a portion of thecompressible objects has an internal pressure (i) greater than about 200pounds per square inch at atmospheric pressure and (ii) selected for apredetermined well pressure, wherein external pressures that exceed theinternal pressure reduce the volume of the compressible object; andwherein the shell has an engineered architecture designed to compensatefor localized strains on the compressible object during expansion andcompression of the compressible object; selecting a drilling fluid to becombined with the compressible objects; blending the compressibleobjects with the drilling fluid to form a variable density drilling mud,wherein the variable density drilling mud maintains a density between apore pressure gradient and a fracture pressure gradient for at least oneinterval of a well as the variable density drilling mud circulatestoward the surface of a well.
 70. The method of claim 69 wherein thecompressible objects have an internal pressure above 500 pounds persquare inch at atmospheric pressure.
 71. The method of claim 69 whereinthe compressible objects have an internal pressure above 1500 pounds persquare inch at atmospheric pressure.
 72. The method of claim 69 whereineach of the compressible objects has a shell, wherein the shell isconfigured to experience less strain when the external pressure is aboutequal to the internal pressure than when the external pressure isgreater than the internal pressure or less than the internal pressure.73. The method of claim 69 wherein each of the compressible objectscomprises a plurality of states, the plurality of states comprising afirst state at atmospheric pressure outside a wellbore, and a second andthird state within the wellbore; wherein the first state has a firstvolume, the second state has a second volume, and the third state has athird volume, wherein the third volume is less than the first volume andgreater than the second volume, and wherein walls of the compressibleobject are under less strain in the third state than in the first stateand the second state.
 74. The method of claim 69 further comprisingblending weighting agents into the drilling fluid to control the densityof the drilling fluid and compressible objects.
 75. The method of claim69 wherein each of the compressible objects has an internal pressureabove about 1500 pounds per square inch at atmospheric pressure.
 76. Themethod of claim 69 wherein each of the compressible objects has one ormore structural members to reduce localized strain.
 77. The method ofclaim 76 wherein the one or more structural members comprises a flange.78. The method of claim 69 wherein each of the compressible objects isan ellipsoid object having an aspect ratio between 2 and 5 when theexternal pressure is about equal to the internal pressure.
 79. Themethod of claim 69 wherein each of the compressible objects is anellipsoid object having an aspect ratio between 3 and 4 when theexternal pressure is about equal to the internal pressure.
 80. Themethod of claim 69 wherein each of the compressible objects has a shell,the shell has an equivalent-diameter-to-wall-thickness ratio in a rangefrom 20 to
 200. 81. The method of claim 69 wherein each of thecompressible objects has a shell, the shell has anequivalent-diameter-to-wall-thickness ratio in a range from 50 and 100.82. The method of claim 69 wherein each of the compressible objects hasa shell, the shell comprises a gas permeation barrier layer andstructural layer.
 83. The method of claim 82 wherein the gas permeationbarrier layer comprises a metal or metal alloy layer and the structurallayer comprises a polymer layer.
 84. The method of claim 69 furthercomprising combining weighting agents with the drilling fluid, whereinthe weighting agents comprise one of barite, hematite, galena and anycombination thereof.
 85. The method of claim 84 further comprisingcombining formates with the drilling fluid to control the density of thedrilling fluid and compressible objects in mud systems and minimize theaddition of insoluble weighting agents that tend to raise viscosity ofthe drilling fluid and compressible objects.
 86. A system associatedwith drilling a wellbore comprising: a wellbore; a variable densitydrilling mud disposed in the wellbore, wherein the variable densitydrilling mud has compressible objects and a drilling fluid, wherein eachof at least a portion of the compressible objects has an internalpressure (i) greater than about 200 pounds per square inch atatmospheric pressure and (ii) selected for a predetermined wellpressure, wherein external pressures that exceed the internal pressurereduce the volume of the compressible object and wherein the shell hasan engineered architecture designed to compensate for localized strainson the compressible object during expansion and compression of thecompressible object; a drilling string disposed within the wellbore; anda bottom hole assembly coupled to the drilling string and disposedwithin the wellbore.
 87. The system of claim 86 wherein each of theplurality of compressible objects has an internal pressure above about500 pounds per square inch at atmospheric pressure.
 88. The system ofclaim 86 wherein the compressible objects comprises a first portion ofthe compressible objects having a first internal pressure and a secondportion of the compressible objects having a second internal pressure,wherein the second internal pressure is different from the firstinternal pressure.
 89. The system of claim 88 further comprising a thirdportion of the compressible objects having a third internal pressure,wherein the third internal pressure is different from the first internalpressure and the second internal pressure.
 90. The system of claim 86wherein the compressible objects comprises a first portion of thecompressible objects having a first volume at the surface of thewellbore and a second portion of the compressible objects having asecond volume at the surface of the wellbore, wherein the second volumeis different from the first volume.
 91. The system of claim 90 furthercomprising a third portion of the compressible objects having a thirdvolume at the surface of the wellbore, wherein the third volume isdifferent from the second volume and the first volume.
 92. The system ofclaim 90 wherein the compressible objects comprises a first portion ofthe compressible objects having a first shape and a second portion ofthe compressible objects having a second shape, wherein the second shapeis different from the first shape.
 93. The system of claim 86 whereineach of the compressible objects has a shell, wherein the shell isconfigured to experience less strain when the external pressure is aboutequal to the internal pressure than when the external pressure isgreater than the internal pressure or less than the internal pressure.94. The system of claim 93 wherein each of the compressible objects isdesigned to compensate for the localized strains and instabilities ofthe compressible objects during expansion and compression of thecompressible object.
 95. The system of claim 86 wherein each of thecompressible objects has a one or more structural members to reducelocalized strain.
 96. The system of claim 95 wherein the one or morestructural members comprise a flange.
 97. The system of claim 86 whereineach of the compressible objects has a shell, wherein the wall thicknessof the shell is varied over the surface of the shell to reduce localizedstrain.
 98. The system of claim 86 wherein each of the compressibleobjects has a shell, wherein the wall thickness of the shell is thickerat the equator of the compressible object to reduce localized strain.99. The system of claim 86 wherein each of the compressible objects hasan internal pressure above about 1500 pounds per square inch.
 100. Thesystem of claim 86 wherein each of the compressible objects is anellipsoid object having an aspect ratio between 2 to 5 when the externalpressure is about equal to the internal pressure.
 101. The system ofclaim 86 wherein each of the compressible objects is an ellipsoid objecthaving an aspect ratio between 3 to 4 when the external pressure isabout equal to the internal pressure.
 102. The system of claim 86wherein each of the compressible objects has a shell, wherein the shellhas an equivalent-diameter-to-wall-thickness ratio in a range from 20 to200.
 103. The system of claim 86 wherein each of the compressibleobjects has a shell, wherein the shell has anequivalent-diameter-to-wall-thickness ratio in a range from 50 and 100.104. The system of claim 86 wherein each of the compressible objects hasa shell, wherein the shell comprises ex-foliated inorganic mineral asre-enforcement or as a barrier to gas permeability in a polymer matrix.105. The system of claim 104 wherein the shell comprises nanofiberre-enforcement in the polymer matrix to achieve the specific propertiesof the wall material.
 106. The system of claim 86 wherein each of thecompressible objects has a shell, wherein the shell comprises a gaspermeation barrier layer and structural layer.
 107. The system of claim106 wherein the gas permeation barrier layer is a metal or metal alloylayer and structural layer is a polymer layer.
 108. The system of claim106 wherein the gas permeation barrier layer is formed external to thestructural layer.
 109. The system of claim 106 wherein the gaspermeation barrier layer is formed internal to the structural layer.